Preparing drywall for earth plastering is generally easier than preparing it for painting, but some steps are a little bit different, and must be done correctly. Here’s a short to-do list for drywall prep:
• Always use a setting type drywall mud such as Sheetrock 90. Pre-mixed (wet) drywall compounds may rehydrate under earth plasters, leading to cracking or unsightly changes in texture along joints.
• Tape and mud joints, but it’s not essential to mud drywall screws. Another pass to level the joint is generally a good idea; be judicious with your mud. Finish drywall to level 2 or level 3 (ASTM C840) depending on how thick your earth plaster coat will be.
• Avoid sanding. If spot sanding is required carefully wipe dust with a wet sponge and allow to dry.
• Prime with good quality paint primer with fine sand in it (around 10% sand). It’s a good precaution to prime just the joints, allow them to dry, then prime the entire wall; this is essential if premixed drywall compound has been used.
• If magnesium board or other hard / non-flexible wallboard alternatives are used, it is essential to leave a small gap (about 1/8”) between boards. Mud this first and allow to dry before mudding and taping the joint. Failure to do this is likely to result in cracks at some joints.
American Clay has an amazing resource describing substrate preparation for natural plasters, available here http://media.wix.com/ugd/9fd6d9_1fad49cfe25f42c4812592293d6e50d4.pdf