The neat thing about drywall compound, also called joint compound or just "mud," is that you can apply it over existing walls to smooth away roughness or for use as a new drywall texture. The compound, which goes on wet, adheres to nearly any porous surface, but in some cases, you need to do some prepping before you can apply the compound.
Drywall compound usually will adhere to a surface painted with a flat, or dull, paint. However, a glossy paint, enamel or paint intended to repel moisture, such as bathroom paint, may not hold the compound as securely. If your painted wall has a glossy surface, you can sand it lightly to remove the sheen or rub the wall down with a liquid sanding product to remove the gloss before applying wet compound.
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If the paint is coming loose now, it could affect a new coat of drywall compound as well. You must scrape away or sand off the loose bits first. Since moisture causes paint to peel away, fix any leaks or moisture problems before applying the new compound. If there are yellow stains, in addition to peeling paint, those are water stains, and they will bleed through the new drywall compound layer. Treat these stains with a wall sealer before applying the new compound.
You can apply new drywall compound right over some types of texture after brushing dust from the surface. Nearly flat textures, such as orange peel and California knockdown, need no sanding if the peaks and depressions of the texture are less than 1/16 inch. For more aggressive texture peaks, you'll have to sand away the tips before applying compound.
Sanding the Drywall
Sanding works better than scraping for the removal of most types of existing texture. Use a drywall sander, and wear a respirator mask and sealed eyewear for protection. The existing texture was made from compound, in most cases, and sanding it down creates clouds of fine dust. A sander with a dust collection bag helps but does not remove all the dust from the air.