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Substrate Preparation


The term substrate refers to the wall surface you will plaster over. You will need to follow the General Preparation and Priming instructions in the following sections for the following surfaces:

  • Painted or sealed surfaces (flat or slightly textured)

    • Also, see adhesion test below.

  • Cement finishes (slick or smooth)

  • Gypsum plasters (e.g.: Structolite®, Gypsolite, Imperial®, Diamond®, Red Top®, Kal Kote®, etc.)


New wallboard and unsealed joint compound: these surfaces also use the General Preparation and Priming instructions, but prior to that, please see

the specific recommendations for joint compound application and preparation below.


Other substrates (e.g.: brick, concrete block, adobe, cement or lime stucco brown coats, and all others not listed) have different preparation requirements.


Please check the American Clay Substrate Preparation document for in-depth information on properly preparing your particular substrate (Download Substrate Preparation Document Now!). You may also call 1-866-404-1634 (toll-free technical support line) for further information.


Wallpaper, ceramic tile, foam, OSB, wood, plywood and paneling are examples of substrates over which plaster is not acceptable. These surfaces must be removed or covered using an appropriate substrate.

Wallboard Preparation


Wallboard is commonly referred to as drywall, plasterboard, Sheetrock®, Gyprock®, gypsum board, blue board, green board, Fiberock® and Quietrock®. Standard everyday drywall is acceptable. Specialty (mold and moisture resistant) drywall panels are not required, but are also acceptable.


Setting-type joint compound, also known as “hot mud”, is recommended for taping and bedding. Examples of setting-type joint compounds are: Durabond® 90 and Sheeetrock® Easy Sand 45 Minute Setting-type Joint Compound. Setting-type joint compounds come in a dry powder form and are mixed with water upon application. We recommend using 45 minute (or longer) versions of setting-type compounds only. All purpose, pre-mixed joint compounds will require further preparation (please see notes below) and may require additional cure

time, please check with manufacturers recommendations.

When all-purpose bedding-type, non-setting, or unknown joint compounds are used to achieve a Level 2 or Level 3 finish (standard); or, if pre-mixed (lightweight or all purpose), topping, and thinned compounds are used to achieve a Level 4 or Level 5 finish, the surface will require additional preparation to avoid peeling and delamination. A sealing primer, like Zinsser® GARDZ®, ROMAN® PRO-999 Rx-35®, or Draw-Tite™ would be used. Please note: the sealing primers may be used with American Clay Primer Sand additive in place of the standard recommended primers (see Step 2. Priming) if that is more suited to your project.


The following are general guidelines to follow:

  • Screws and fasteners DO NOT require joint compound

  • Seams DO need tape and joint compound

  • Extra passes may be needed at corner bead or where additional leveling is necessary

  • DO NOT sand joint compound

  • High points DO need to be scraped off or knocked down with a putty knife or scraper.

  • Use mesh tape with setting-type joint compound. If you use paper tape, be sure the work is well done and that no air is trapped behind the paper, as it will cause the plaster to delaminate when it dries.



General Preparation


Complete any necessary "General Prep" steps required to bring the substrate to a relatively flat, dust free, well-bonded surface.


The following are general guidelines to follow:

  • Scrape off any loose or flaking paint or other surface material.

  • Clean and fill any depressions deeper than 1/16" for all plaster applications except Porcelina finishes. Clean and fill any depressions deeper than 1/32" for Porcelina plaster applications. Use a filler that bonds to the substrate.

  • Knock down high points higher than 1/16" for for all plaster applications except Porcelina finishes. Knock down high points higher than 1/32" for Porcelina plaster applications.

  • Lightly sand any high-gloss paint or glossy sealed surface with 150-grit sandpaper to provide a tooth for the primer.


*Warning: If you scrape, sand, or remove old paint, you may release lead dust. Lead is toxic. Exposure to lead dust can cause serious illness, such as brain damage, especially in children. For more information, visit:


  • Remove any dust with a vacuum or clean with a tile sponge.

  • Wash sooty or greasy surfaces with a TSP (trisodium phosphate) substitute cleaner of your choice. Let dry completely.

  • Protect floors with drop cloths or plastic. Tape all adjacent surfaces. Keep tape 1/8" away from the surface being plastered, so tape does not pull off any plaster when tape is removed.

  • Maintain temperatures between 45 and 90 degrees Fahrenheit for three days before, during, and three days after application.

  • Maintain humidity levels below 50% humidity during application to facilitate drying. For humid climates, dehumidifiers and fans can be used as a drying aid.

Adhesion Test for Paint


For both newly painted walls and walls with many layers of paint, it is good to check the paint adhesion. This simple test could save you labor by identifying potential substrate weakness early in the process.


Using a utility knife, make several light cuts in the paint three to five inches apart, then apply a strip of white masking tape perpendicular to the cuts. Press the tape firmly and then peel it off. The paint should remain firmly attached to the wall. If it does not, check with your local paint supplier for recommendations on making the surface sound. Testing on areas where seams exist is recommended.



Please continue with the guided Online Workshop steps below:

Online Workshop

Please note: The guidelines outlined in these instructions are provided as a source for general application procedures. We are not responsible for the results of your project. The only way to ensure a proper application is with due diligence on your part. If you have questions or are unsure of anything, please call 1-866-404-1634 (toll-free technical support line) for further information.

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