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On new construction, you have to prime. On remodels, where there is normal latex paint, you don't have to prime. Our plasters will stick to previous paint, as long as it's not damaged.
Use a water based / acrylic / latex high grade primer to prime the drywall. Primer should be used on other substrates, such as wood, glass, metal or almost anything that latex primer will bond to. Not only do you want the primer to adhere extremely well to the wall, but if there is ever a leak from plumbing or rain, the primer will create a modest barrier between the plaster and the leak. Mask before you prime. (See best masking practices on page 3) Also, when using most stain blocking primers in smaller confined areas with little ventilation, consider wearing a respirator (not a dust mask). Some primers have styrene, carbonized chlorine and ammonia. Ask your paint store for low VOC primer. With Veneziano, good priming is crucial, especially on new construction. Since you will be applying thin layers of plaster, make sure there is absolutely no taping mud/gypsum mud showing through. This will crackle or discolor the plaster and make for very poor adherence. In fact, priming twice is best.
After priming, if you want, you can lightly scuff the walls for a little more 'tooth'.
REMODELS AND OLD WALLS
Light orange peel texture is fine as long as you use have a thick first layer or three layers of plaster instead of two (only burnish on the third coat). For thicker texture, you should skim coat drywall with plaster/mud, then sand until the walls are reasonably smooth. For thicker 'cottage cheese' walls, sand with at least 80-grit or lower sandpaper, preferably with an orbital sander. Then dust off (clean) and prime the walls with an above-mentioned primer. You can skip priming if the walls are relatively new and are well painted. Just remember to scuff walls to give a little extra tooth to the plaster. Remember that even if the existing walls aren't perfect or totally smooth, and you're not going for a shiney smooth finish with plaster, your walls will still look natural and the way they're supposed to be.
Use your best judgement on applying over really old walls in bad shape. Try a small area first. Let it dry and make sure it sticks. Veneziano will breath with the wall. It won't allow for mold growth.
PLASTERING OVER WALLPAPER
If there is wallpaper, strip it, then prime the wall. If you are willing to play it less safe; be certain that it is firmly attached to the wall before applying plaster over it. Find a corner of the wallpaper and try to strip it dry. If it comes off with ease, is old and peeling, or has several layers it is best to strip it. If the wallpaper is on the ceiling; pay special attention. It is always best to strip the ceiling of wallpaper; gravity will not be your friend. If the wallpaper adheres well to the wall, prime it with an oil-based primer, such as Zinser's or Killz oil based primers (these primers can be messy and hard to clean). If it bubble when it dries, cut out all the bubbled paper out of the wall and re-prime that spot. Afterwards, prime with latex primer so the plaster can stick to that. Consult your local paint store for further advice. Make really sure that you explain to them what you want to do as long as there is latex on the last coat before the plaster. In any case, stripping wallpaper is the best practice.