Frequently Asked Questions
Do you have a question regarding American Clay? See below for answers to some of our most common questions. Click on the question to view the answer.
For Frequently Asked Questions regarding our newest plaster line - Forté, please click here.
Substrates and Wall Preparation
Bathrooms, Showers, and Humidity Buffering
Sealing Plaster and Backsplashes
Cleaning, Repair, and Maintenance
Color and Pigments
Frequently Asked Questions & Answers
Is American Clay a paint or a plaster?
American Clay is a plaster. We do not sell a clay paint. It is applied with a hawk and trowel, not a paint brush or paint roller. It can be compared to Venetian plaster and other veneer plaster type applications. The average cost of the material can range from $1.35 to $2.00 per square foot depending on color and finish choices (costs do not include labor, which vary project to project based on many factors specific to each job). The material comes in dry sacks to be mixed with water on site, not in liquid gallon cans like most paint products do. It is applied to many different surfaces like paint (including drywall, painted surfaces, etc.). The product is derived from natural sources, is zero-VOC, and is made of a proprietary blend of sand aggregates and clays sourced entirely from the United States.
What is the composition or ingredients of your clay plasters?
American Clay is a blend of post-industrial sand aggregates, clays and pigments. The post-industrial aggregates are comprised of rock and shell waste crushed into sand. American Clay plasters do not require high-energy kilns or reactors like other plasters and acrylics. Materials are also sourced from within the United States.
I ordered and received a few sample squares and noticed that when touched, it seemed like some of the plaster dust rubbed off. I have a 7 month old baby and am sure whatever is on the walls will make it into her mouth soon. I wanted to check about the safety of the product for children, get a little more information about what it is made from, and see if the dust when rubbing is something that will get better over time.
-Answer by Croft Elsaesser, President and CEO of American Clay Enterprises, LLC:
I will reference my own experiences and having used it as all three of my boys have grown up in rooms with clay plaster. In their rooms, I only used the clay plaster on top of a sanded primer so that it was the clay, marble sand, and the mineral pigments. My middle son liked to scrape small amounts off of the wall next to his bed and eat the material. He luckily grew out of doing that, and all I had to do was repair those areas. The rest of the damage was the usual hurling of toys and the occasional crayon or pencil drawing. A very well wrung out sponge took care of this for the most part, with major nicks repaired like I would a nail hole from a picture. The residual crumbles and sand that was pulled from the surface was easily vacuumed or swept away with little concern on my part.
The material is a blend of clays, marble sands, and natural mineral pigments. The biggest concern is the small amount of crystalline silica at a concentration of less than 2%. This would be far less than what is available in a typical sand box or sand covered playground which is usually 30% or higher.
Finally, samples are difficult to fully prevent dusting from the exposed cut edge as this area is rubbed and loosens the clay bond to the sand. If a sponge with very little water is used to compress this area again the dusting will stop until it is agitated with rubbing. A wall will usually experience less disruption and you should see less sand falling away from the surface. This sand is calcium carbonate, and possesses very little risk for any that does fall.
Do you recommend that this product only be applied by trained/experienced persons?
Our product can be applied by just about anyone. We have experienced plasterers of 20+ years use our plasters, but on the other hand, we also have first time handy homeowners and do-it-yourselfers use our products successfully every day.
Is priming really required?
Priming is an important part of our application process, depending on the specific substrate you are planning to use American Clay over. On old painted surfaces or new drywall/gypsum board surfaces, priming is crucial and ensures a proper bond between the wall surface and the clay plaster. American Clay recommends a “sanded primer” in most instances, a mix of American Clay Primer Sand additive and an approved, tested, paint primer. You can find the list of approved primers by clicking here. The sanded primer leaves a surface that is rough like sand paper – this surface allows for a mechanical bond between the plaster and the wall surface.
Is Up & EZ! really needed?
We highly recommend the Up & EZ! binder for every Original plaster finish application. Up & EZ! is not required or recommended to be used in Forté plaster finish applications. This is part of our Original plaster system; most will use a primer with our sand additive and then plaster over this with the clay and Up & EZ! binder. This adds durability and impact resistance to the plaster which will help create a longer lasting product over time. Up & EZ! binder should be used in both coats of plaster or in the single coat of Enjarre™ plaster when using our one-coat application method. Although, if you don't add Up & EZ! binder to the base coat, you cannot only use it in the second/finish coat due to bonding issues.
If you choose to go without the Up & EZ! binder, please make sure every square inch of your surface is prepared properly. Ensure the sanded primer is on all surfaces and double coat any outside corners or other vulnerable areas. At that point plaster can be applied (with or without the binder).
What is the typical wall surface to apply American Clay over?
The most common wall surfaces that American Clay is applied over include old painted walls and newly taped and mudded drywall/gypsum board. These surfaces are easy to prep for American Clay application: in its most basic form you would prime using a recommended paint primer (click here for the complete list) mixed with American Clay Primer Sand additive. This would be followed by the plaster(s) of your choice. For complete instructions in preparing your surface, click here.
What can’t American Clay go over?
Ninety percent of all wall surfaces will be new drywall with joint compound or old painted surfaces, requiring general preparation (basic cleaning and masking) followed by a coat of a recommended paint primer (click here for the complete list) mixed with American Clay Primer Sand additive.
A few other substrates are not suitable for American Clay application, and must be removed or covered with drywall or other suitable surface prior to plastering. They include: OSB, plywood and all other forms of wood, paneling, wallpaper, ceramic tile, foam insulation or foam (poly) surfaces.
Other substrates may require a brown coat of plaster (finished rough like sand paper) to level and/or strengthen the wall system. These include: cement block, straw bale, adobe, etc. Preferably, the plaster would be lime or cement based, which would require no further preparation.
Does your product work well on wood paneling?
We would not recommend applying our plaster over wood paneling. You will have delamination, cracking and other problems because the paneling tends to be very flexible and expands/contracts over time. Although, most of the time, drywall or some other more suitable substrate is directly underneath this type of material. If it is drywall, you could simply follow our normal application instructions after removing the paneling from the walls.
Is there any advantage of using Blue board instead of regular gypsum board/drywall as a substrate to apply American Clay? What about other specialty wallboards?
Other than your wallboard being a bit stronger because of the denser Blue board, there is virtually no benefit to using this material instead of standard drywall. All preparation work will be the same regardless. Specialty wallboards (like mold or mildew resistant boards) are not required, standard drywall is just as appropriate to use.
I saw mention of your product as an alternative to wallboard which has quite a polluting manufacturing process. Going over your website, you suggest applying over drywall which defeats the purpose. What ecologically sound method do you suggest for using your product in a new build?
Unfortunately, drywall (and its manufacturing processes) is the most commonly used wall surface in the building industry. We provide guidance for this material as well as others to ensure proper installation of our materials. Although, if you are interested in alternatives, you could use a lime based plaster over lath or a new recycled drywall material – ReWall. ReWall products feature 100% recycled content, moisture, mold and abuse resistance, and no VOCs. The recycled content is shredded and compressed milk and juice cartons, giving the user a sustainable and superior performing panel. Find out more information by visiting the ReWall website by clicking here.
Can any of your products be applied over heavy texture?
When you have a heavily textured surface, it can make it a bit trickier to install our product over the top of it. Some have tried multiple coats of our Enjarre™ plaster, which goes on thicker than all of our others (but this is more costly). You can also try scraping all of the high areas of the texture then follow our normal application procedures (but this involves more labor). You may also go over the top of your surface using a cement or lime based plaster that would fill the texture -- following all of that plasters manufacturers’ recommendations -- and then go over that with American Clay.
Can I put American clay on painted block walls? Is there any additional work to do on block as opposed to drywall?
You can put American Clay on top of the painted blocks with similar application procedures. You would need to prime the walls using a recommended paint primer mixed with American Clay Primer Sand additive. Then, you would apply the plaster. You will have three options depending on your walls -- specifically if the grout lines are very deep (more than 1/16"):
• If your grout lines are less than 1/16" deep: you can follow the same procedures as stated for painted walls or new drywall. This would be followed by the plaster(s) of your choice.
• If your grout lines are more than 1/16" deep but less than 1/8" deep: you will most likely require a coat of Enjarre™ plaster to fill the grout lines first. This would be followed by the Original plaster(s) of your choice.
• If your grout lines are more than 1/8" deep: you will need to fill using a cement or lime based plaster (finished to a rough texture like sand paper). This would be followed by the plaster(s) of your choice without priming, as long as a cement or lime based plaster is used. If a gypsum plaster or other material is used, please consult our Substrate Preparation document (click here) for specific instructions to that material.
We would love to use the American Clay product on our basement walls because the texture is just what we are looking for. They currently are just cinder block. Can I put the product directly on top of that or would there be too much moisture? We live in Alabama with very hot summers and have a creek running thru our backyard. I reviewed your humidity document, but wasn't sure about applying the product directly to the block wall.
American Clay plaster can go on top of cinder block walls. Although, if water is coming through the wall from the outside because it has not been properly sealed you will need to ensure you have a barrier of some sort installed -- if not the clay will not hold up over time. This barrier can be from the outside (optimal) or from the inside. You will want to speak with the manufacturer and/or contractor to ensure that a primer would stick to whatever surface you may need to create that barrier.
Will your products adhere to a plastic corner bead? Is there a product or procedure to ensure correct application on these corners?
Plastic corner beads will work fine. Make sure to use sanded primer on these surfaces and use two coats. Also, a tip would be to take a 150-grit sand paper and rough up the corner bead a bit prior to applying the primer. This will help ensure adhesion. Be sure to wipe any dust prior to priming. Then, follow general application procedures.
Does your product work well over molding?
The plaster cannot go over molding. American Clay does not work well over substrates which move or expand/contract. The plaster will end up cracking or delaminating off the surface.
I am in the process building a small passive solar home using new ICF construction. I recently learned of American Clay and am very excited at the possibility for using this for interior wall surfacing. I understand that American Clay can be used over drywall but I am curious to learn whether there is a method that could be used without the drywall. Is there a coating procedure that can be used directly on the ICF?
Unfortunately, when using ICF's you cannot go directly over the top of this material with American Clay. You must provide some other more suitable material for the clay to be applied to. You can use drywall and follow our typical application instructions by taping/mudding the seams, priming with a sanded primer, and finishing with the plaster application. Or, you could also use a plaster brown coat to cover the ICF -- a lime or fibered cement based plaster finished to a sandy texture ("brown coat"). This will stabilize the ICF while providing a base for the clay plaster. Then, proceed directly to the American Clay plaster application.
Can American Clay be applied on cement board?
There are many types of cement board available to use, however, American Clay recommends using HardieBacker® cement board. Durock® cement board can be used, although, the propensity for this board to crack over time may cause issues with the clay plaster after installation and/or will require further preparation prior to clay installation. American Clay strongly recommends against using Durock® cement board for these reasons.
Once the cement board has been installed onto the wall, follow the manufacturers guidelines (below) for taping and bedding the joints and seams:
HardieBacker®, JamesHardie Manufacturers Guidelines:
Fill all joints with mortar (Latex or acrylic modified thin-set or Type 1 mastic) and immediately embed 2 inch wide high-strength alkali-resistant fiberglass tape in the mortar and level.
Alternatively: American Clay also recommends using setting-type joint compound (also known as "hot mud") with fiberglass tape to tape and bed joints and seems. All purpose, pre-mixed joint compounds are not suitable. Examples of setting-type joint compounds are: "Durabond® 90", "Sheeetrock® Easy Sand 45 (or 90) Minute Setting-type Joint Compound", and “Rapid Set® OnePass® Wall Repair Material and Joint Compound”. Setting-type joint compounds come in a dry powder form and are mixed with water upon application.
Durock®, USG Manufacturers Guidelines:
*Please note: even with these guidelines, American Clay cannot guarantee that the Durock® cement board will not crack over time and show through the plaster coat. Warranty of the product will be voided.
Fill all joints with latex-fortified mortar or Type 1 organic adhesive and immediately embed 2 inch wide high-strength alkali-resistant fiberglass tape in the mortar and level. Let dry completely.
Additionally: Using the same mortar or adhesive, completely coat the entire board. Let dry completely.
After the above guidelines have been completed to prepare the cement board: prime the entire surface with an "Approved" paint primer mixed with American Clay Primer Sand additive. This will also help to even out any suction variations between the joint compound and the cement board.
I am planning to use American Clay in my home. I'm a little confused about using it on the ceilings. Is it normally just applied to the walls, and the ceiling painted – or, do you apply to the ceiling as well? Are there special application instructions for ceilings?
The ceilings can be finished in a number of ways. You can use American Clay throughout the home on the ceilings and walls for a perfect match. The application instructions are the same for the walls or ceilings, it tends to just be a bit more difficult to plaster the ceiling as you are working over your head and against gravity. You can use our Forté White or Sugarloaf White colors for the ceilings or continue with the same color as the walls -- they all look great.
Or, you can paint the ceilings and use American Clay on the walls only. If you go this route, please ensure the paint and clay go well together prior to the project. We have had customers choose a ceiling color after the fact and it drastically changed the way the plaster color ultimately looked. Since most contractors work top down, it can cause a ceiling repaint to be much more work with taping off and protecting the finished clay plaster wall.
Can American Clay be plastered over popcorn ceilings?
All popcorn ceilings must be completely removed prior to applying the clay plaster. The weight of the clay will end up pulling the popcorn ceilings down causing a huge mess if this is not taken care of first.
Can American Clay plaster be used in a bathroom with a shower? Is there a specific product that works better?
American Clay plasters work beautifully in a bathroom with a shower. Our plasters naturally absorb moisture from the air and later releases it back as the air dries out (humidity buffering). This leads to less water vapor on the surface of the wall which can lead to mold or mildew growth (like on many painted surfaces in bathrooms). This process is also demonstrated in a short video (watch by clicking here) or in a handout (read more by downloading here).
The plaster will be fine everywhere except inside the shower unit where it will be exposed to water contact continuously -- this will erode the plaster away very quickly if done inside the unit. Areas above the shower, near the shower, or the backsplash behind the sink are fine when plastered with American Clay. If soap residue or water spots occur, they can be cleaned using a damp tile sponge with a light amount of pressure.
The choice of plaster will not matter as they all react with moisture/humidity in the same ways. They are basically the same products with minor differences in aggregate sizes only.
Can American Clay plaster be used inside a shower? What if we seal the plaster?
No. Our plaster is not meant to be used in an environment like this. The shower will leave the plaster exposed to continuous water contact -- this will erode the plaster away very quickly. Areas above the shower or near the shower are fine when plastered with American Clay. If soap residue or water spots occur, they can be cleaned using a damp tile sponge with a light amount of pressure. Over time, even if our plasters are sealed as well as possible, the material will degrade back to the original surface causing damage to the room.
What is humidity buffering and at what humidity level does the clay try to hold the room?
Our plasters naturally absorb moisture from the air and later releases it back as the air dries out (humidity buffering). This leads to less water vapor on the surface of the wall which can lead to mold or mildew growth (like on many painted surfaces in bathrooms). American Clay does help regulate humidity in a space, but it really does not regulate it constantly to a specific level. The clay helps to lessen the swings of humidity changes within a space over time. The fluctuations are dependent more on the volume of space vs. the amount of clay being used. More clay being used in a specific space would help the regulation as compared to less clay. This could be by lowering humidity in highly humid environments (ex: Florida, New Orleans) or helping to increase humidity in low humidity environments (ex: New Mexico, Arizona).
My husband is applying American Clay right now and he won’t believe me that after the compression stage there won’t be wall dust. He insists I look into a sealer. Will it change the color of my walls? What is your take on sealers?
American Clay almost always recommends against sealing the wall. This impacts the repairability factor as well as the breathability of the surface. If applied properly, and compression is done, your walls will not be chalky, dusty, or sandy at all -- you are 100% right. If you do seal the surface, the color will change (usually a bit darker in shade) so please test prior to using on the entire surface in case you don't like the shift in color. Keep in mind: once the surface is sealed and a repair needs to be done, the sealer must be broken down and removed first. See plastering over a sealed wall (next question below).
If my American Clay walls are sealed, is it still possible to apply clay? Or, is painting my only option?
If the clay walls are sealed, you have a few options:
1. Remove the sealer using a vinegar and water solution (1 part white vinegar to 3 parts water) and apply one layer of new plaster. You will need to vigorously scrub with a sponge using the solution.
2. Prime over the sealed surface and apply 2 new layers of plaster or 1 layer if using Enjarre™. Think a brand new application and follow our information for applying plaster over a painted wall/new drywall.
3. Paint over the sealed surface. The texture will remain, but you will be able to use the paint and color of your choosing.
We are doing a small kitchen renovation that will include refinishing the walls behind a sink and behind a stovetop. We are considering using your product on those walls rather than a tile or granite backsplash - if it is appropriate. Since these are areas that would have to tolerate splatter and grease, we want to be sure that they can be easily cleaned. Would your product work in these locations?
American Clay can be used in the backsplash areas around a sink or stove top. The area around the sink would work beautifully with the product being easily cleaned using a damp tile sponge every so often. The soap residue and water would not be an issue here. Although, around the stove top, we typically will recommend tiling because the oils from cooking are much more difficult to remove. The area would probably see discoloration and might need repairs often. We have seen customers use it in these areas and the results can be good. If you do decide to use American Clay around the stove top we would recommend sealing the area with a water based acrylic masonry sealer for additional protection. Any of the plaster finishes would work just as well as the others -- the choice should be made on personal preference.
I have a backsplash that I'm redoing and I'm planning on installing tile over a small section that currently has American Clay over it. Is it okay or do I have to remove the clay before I install the tile?
You will need to remove the clay first so that you will not have the tile falling off later. The clay will not support the weight of the tile.
Is there a way to make the clay plaster walls easier to clean?
We typically recommend that you apply the product as directed, without sealing the surface, so that the material is always easily cleaned. If you have dirt, grime or stain issues that develop you can take a damp tile sponge and buff out almost any damage relatively easily. If sealed, the wall surface must be unsealed prior to cleaning or fixing a damaged area. Remove the sealer using a vinegar and water solution (1 part white vinegar to 3 parts water). You will need to scrub with a sponge using the solution, this may take multiple passes.
We have concerns regarding the process of cleaning American Clay once it is applied. Do you have information regarding recommended cleaning procedures?
The walls should not need to be wiped down as often as a painted surface as they will resist dust from sticking to the surface. But let’s say you spilled something, a child marked up the surface with a marker, or have soap scum from near a bathroom sink -- the walls can be cleaned by using a damp tile sponge to remove a micro-layer of material from the surface. The damaged area should return to normal after doing this small sponging step. This process is showed in a video, watch by clicking here.
If you have a stain, a solution of 25% white vinegar and 75% water is used to help remove the marks. Sometimes, simply spraying the wall with the solution helps to get rid of deep water spots or weak stains – it may take two or three passes. If the stain is really strong, use the solution when sponging the area to help clean the wall. Some stains may require a patch repair of plaster to bring the surface back to normal.
The only thing you would need to be careful of is having splashing over and over near an American Clay surface. The walls will not hold up to constant water contact day after day. Behind a backsplash of a sink is okay but inside the shower unit would create too much moisture. Also, if you are planning on using the clay behind the stove, you may want to seal that area because the constant splashing of oils and food will degrade the surface as you clean it every day.
My wonderful 4 year old decided our bedroom wall was an appropriate canvas for her line drawing with a red marker. What would be your suggestion for removing the artwork?
The best approach is a solution of 25% white vinegar and 75% water. Using a tile sponge, dip in the solution and wring the sponge completely out. Dab the marker area, evenly wetting the area. Continue until you have removed the marker. Then, using the sponge wet the plaster and re-smooth using a trowel. If you don't have a trowel, you can use an old expired credit card or gift card (sand off any painted decoration on the gift card first) and then let this dry. If there is a lighter area in color, use the sponge with water only, very well wrung out and lightly buff the area. Try to expand the area about 6 inches around the mark for a more even and less visible repair. It sounds like many steps, but goes very quickly.
I want to install a few pieces of hardware that require a bracket and screws over my new American Clay wall. I am nervous that the clay will crack. What are the chances that this will happen and how can we safeguard against it?
The clay is more flexible than traditional plaster products. Typically, you will not see cracking in the walls unless the substrate or wall surface below the clay is moving as well. If you can protect against the entire wall moving you should be fine. Although, testing slowly at first will be the best bet in case something does go wrong so you can stop before major damage occurs. A standard nail or screw being driven into the wall surface should not do damage to the clay around the hole.
Can I go over a compressed wall with a new coat of American Clay?
You should have no concerns going over a compressed wall with another coat of plaster. You should pre-wet the surface using a misting bottle prior to applying a new coat of plaster. Then, once the new coat of plaster is dry, finalize it by using a proper compression. Once this is done, you should have a really durable and stable wall for the future.
If you wish to use a Forté plaster over your old American Clay surface, you will need to prep the wall first using a bonding adhesive. For more information, see plastering over an Original finish using Forté plasters.
What would happen to the plaster if the house flooded?
If your house floods, the walls will most likely deteriorate where they come in contact with water. Our plasters are not meant to stand up to constant water damage. This is why we don't recommend American Clay in areas where water will constantly run over the surface (inside the shower stall in a bathroom, behind a plant that will be watered daily where water will spill, etc.). Even sealing the surface will not help the material hold up to this type of damage. An exterior grade plaster would probably hold up better in this scenario.
How do I remove your product from my walls? Is it possible to paint over it with a water based paint? How will that effect the moisture emitting nature of the product?
You can remove the product from the walls in a few ways:
1. You can try to paint over the walls using a high quality paint and primer in one. This will keep the texture of the surface but allow you to paint in any color. But this does not ensure that future layers of paint, etc. will stick.
2. You can rehydrate the clay and scrape the product off the wall completely. Typically, it is only about 1/16” thick so this can work.
3. You can ensure proper adhesion for future layers of paint, wallpaper, or plaster by sealing the surface with any water based acrylic masonry sealer. Once dry, you can use any product you wish.
Once the plaster is painted or sealed and then covered with another material, the breathability and humidity buffering qualities will be completely eliminated.
I have an American Clay wall that has a black soap finish. Can I just prime over the walls and have a clean slate to work with?
To use our plaster over the top of a black soaped American Clay wall: first, use a fairly damp sponge to remove most of the black soap. Go over the top of the wall and roughen up the surface while removing the soap that has penetrated. This may require multiple passes - be sure to let the plaster dry completely between each pass. This allows our plaster to be applied directly over the top of the surface once dried. If you were painting: you would still need to seal the surface using a water based acrylic masonry sealer over the wall prior to painting.
I would like to change the color of one of my rooms that has American Clay. Can I put a dark color over a light color?
The great thing about American Clay is that there is no bleed through with color. You could put white over black without an issue, all in a single coat. As long as you have not sealed the American Clay walls, you can go directly over them with a single layer of new plaster in any color and finish. The new layer will completely cover your old plaster and there should not be a bleed through of color (like during many painting applications). A tip: spray the wall with a bit of water using a misting sprayer prior to applying the new coat to give you more working time.
If you wish to use a Forté plaster over your old American Clay surface, you will need to prep the wall first using a bonding adhesive. For more information, see plastering over an Original finish using Forté plasters.
Can I add American Clay color pigments to a cement plaster, paint, stucco, or any other similar material?
Our natural color pigments can be used in many materials other than clay plaster, but the colors will be different based on the materials that you use. We don't offer specific guidance on this type of application so you will want to test first to ensure the pigments are doing what you want them to do in regard to color and performance. We have had customers use or color pigments in numerous plasters and paints and have seen great results.
I see your "list of our standard colors that contain pigments that will degrade when combined with lime" but is there a list of lime safe pigments for washes?
All of our colors are safe when lime washing. The only thing you must watch out for is letting material sit wet for longer than 12 hours. Any of the pigments listed that contain ultramarine blue pigment can degrade when left to sit for over 12 hours. If you mix what you will apply in a single day you will be fine. Any proper lime wash application should not take this long to dry so there will be no issues if you simply follow that one warning. The list of pigments that contain ultramarine can be found by clicking here.
I want to use American Clay as a surface for a fireplace, but I have read that using lime-based products will leach the color out. Is it bad to use lime plaster as a substrate?
Putting American Clay on a lime surface will not effect the color. A lime plaster left rough like rough sand paper is a wonderful substrate for American Clay as long as you let it cure completely. You may go directly on the plaster with no other substrate preparation. Lime and clay work very well together, whereas clay and gypsum do not. You will not have peeling or color issues so long as you let it cure for the full time. Please consult the manufacturer’s recommendations for cure times.
Can American Clay be used on the exterior?
No. Unfortunately, our plaster is meant for interior use only. The plaster we sell will not stand up to the weather on an exterior surface. Over time, even if sealed as well as possible, it will degrade back to the original surface.
Can lime wash be used on an exterior surface? In my area of the country at this time of the year the temperatures get pretty hot. Does you recommend a certain temperature and weather range?
Our lime putty is used mostly in a lime wash (lime putty diluted with water). This can hold up outside as an additional coating over many kinds of surfaces like bricks or lime plaster. If you plan to make your own lime based plaster it will depend on how your mixture is made and the application procedures you use.
With lime wash, it is important that you watch out for weather conditions like rain, sun and wind. Although, you can still work in these conditions with precautions. If it is hot, you can plan your work day to work as much as possible in shady areas; or, if you have to apply the lime wash in direct sunlight, pre-wetting of the surface and keeping the material damp will be required so it does not flash dry and chalk or dust on the surface. It may require more than one person so all runs smoothly or just working at a slightly slower pace.