This should always be allowed to dry thoroughly before it is decorated, unless you are going to use emulsion paint designed especially for new plaster. Unlike vinyl paints, this will allow the moisture to continue to evaporate.
The time needed for drying depends upon its thickness as well as the warmth and humidity of the room.
As plaster dries it changes colour from dark to pale pink. When it is dry, any solids which appear on the surface (efflourescence) should be removed. A good way of doing this is to rub the surface with an old piece of hessian.
New plaster will need to be primed. The type of primer used will depend upon the final decoration required. If you plan to use oil-based paint, an alkaline-resisting primer should be used. For water-based paints, a diluted first coat is used to prime.
For wallpapering, the new surface will need to be sized. This is done either with size, or by using a diluted version of the wallpaper adhesive. It is usually best to use an adhesive containing a fungicide. This is particularly important if you plan to hang vinyl paper since it effectively seals the surface preventing it from breathing.
The preparation required will depend upon the condition of the surface. If it is clean, dry, and sound it will require no special preparation.
However, if the surface is powdery or there is other evidence of old distemper residue, it should be dusted off and sealed with stabilising solution.
Paintwork should always be washed down first. This is necessary to remove traces of dirt and grease, which would otherwise cause failure of subsequent decoration.
Gloss painted surfaces must also be rubbed down to provide a key. They will require further treatment if you intend to use anything other than oil paint.
For emulsion paint, a good trick is to apply an undercoat, or matt finish primer, since these will adhere to the gloss and will accept the new paint. If you intend to hang wallpaper, you should hang lining paper first.
If the existing paint is powdery, it may well be distemper. Preferably, this should be removed before redecorating. Dusty or powdery surfaces should be sealed with stabilising solution.
New plasterboard is usually skimmed with a thin coat of plaster or a slurry of filler. In this case, it may be treated as new plaster.
If you intend to decorate the plasterboard directly, you should prime the surface. For emulsion, use a diluted coat first to prime. For oil paints, and wallpapering, prime with all-purpose primer.
Distemper coated surfaces
This is not suitablerede to receive modern paints or paper and should be removed.
The process for its removal is similar to that for stripping wallpaper.
Previously painted wallpaper may be washed and repainted as for painted plaster, provided it is sound.
Otherwise, as for all the other wallpapers, it should be stripped. It is possible to paint over wallpaper but with some exceptions. Always test a small area first as the liquid content of paint may cause the paper to lift.The dyes used to create the pattern in the paper may bleed through new paint. In this case you will need first to paint the surface with aluminium sealer.
It is not recommended to paint over vinyl paper as this is designed to prevent anything sticking to it.
If you intend to hang another wallpaper it is always best to remove totally the existing paper. It is not good practice to hang paper over paper, except in the case of lining paper or previously painted paper in sound condition.