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 willadhere 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.