Before you can decorate over newly finished plaster, you must allow it to dry out completely. Depending on the size of the area plastered, this could take several days. If efflorescence is forming on the surface, it is still drying out. Wipe any efflorescence away each time it appears. Once it stops, the plaster should be dry. Emulsion can now be applied to the walls with or without a primer coat. If you plan to paper the walls, apply a coat of Size (a product formulated to stop paste soaking completely into the plaster) and allow it to dry. The plaster is now ready to be papered. If you are tiling over the new plaster, no preparation is needed once the plaster is dry.
Fill and smooth any minor cracks or holes and allow the filler to dry properly. If the old plaster is patchy, you can apply an all-purpose primer to stabilise it. Once this is dry, the plaster should be ready to paint, paper or tile over.
If you are not planning to apply a skim layer to plasterboard, you will need to fill any joints and cover with joint tape. If you plan to paint the walls, a skim layer will give a much better finish. If not, at least make sure paint any nail heads with a Zinc Phosphate primer. If you are planning to paint the plasterboard, prime it with at least one layer of general-purpose primer or a thinned coat of emulsion. If papering over the plasterboard, seal the surface with an general-purpose primer. Allow this to dry completely before applying a coat of Size.
If you are painting or papering over existing plaster which has a layer of paint on it, you need to prepare the surface carefully. Assuming the emulsion paintwork is in good condition, rub it down well with a sugar soap or detergent solution. If you are painting a light colour over dark emulsion, a coat of primer will be needed. If painting over gloss paint, rub it down with water and some medium grade wet and dry sandpaper. This will key the surface and allow the new coat to adhere properly. Apply a coat of primer and allow this to dry. If you are papering over painted plaster, key the surface with a medium grade abrasive paper and then apply a coat of Size. Allow this to dry completely.