How to Repair Damaged Plaster Before Decorating
I can hear my dad saying to me time and again, all those years ago, ‘The finish is only as good as the prep work that goes into it!’ How right he was! You can expect to spend two days preparing and half an hour painting, rather than the other way round. Above all, you should repair and seal all old and damaged plaster before decorating.
A cracked, unstable or damp-damaged plaster surface will spoil any paint that is applied to it, so be sure to remedy any problems before you start decorating. If you paint over new plaster that has not dried out properly, dusty old plaster, existing flaking paintwork, a greasy surface -usually in a kitchen – or a damp wall, the new paintwork will start to flake in a relatively short time.
New plaster is porous so it must be sealed (primed) before it can be painted, whether you intend to paint the plaster directly or to cover it with lining paper first. As it must be completely dry before this can be done, allow at least 4 weeks after plastering before you start work.
Use either diluted PVA adhesive (5 parts water to 1 part PVA) or diluted emulsion paint (1 part water to 8 parts emulsion) to prime the wall. Apply liberally.
Old Plaster and Paintwork
Old, bare plaster often has a dusty coating that comes off on the hand. Such walls need to be coated with a stabilizing primer to prevent fresh paint from flaking off; water-diluted PVA adhesive is ideal. If the surface is not dusty, you can seal the plaster with diluted emulsion paint.
Old paintwork that is flaking needs to be scraped back with a paint scraper tool until only sound paintwork remains. There is no short-cut here, just hard graft! When it is in a good state, coat the wall with stabilizing primer before decorating. If the wall was painted previously and the covering is sound, just wash it all over with diluted sugar soap, then rinse with clean water. You can then paint it directly with emulsion.
Dealing with Damp
If you just paint over a damp area the new paint will flake off in weeks, so where damp is suspected you must find the cause and remedy it before you do anything else. Damp plaster is normally caused by leaking or faulty rainwater goods (gutters or down pipes), or penetrating or rising damp caused by the external ground level bridging the damp-proof course. Lowering all external ground levels to 100-150mm (4-6in) below the damp-proof course, and repairing or replacing leaking or damaged rainwater goods should solve most damp problems. Penetrating damp can be caused by a crack or a porous wall – fill cracks and apply silicone water repellent or paint with masonry paint.
Even after a damp problem has been rectified, a dried stain is often left behind. This must be sealed, otherwise the stain will reappear through the new paint after it has dried. Stabilizing solution and aluminium primer sealers are the best products for covering an old damp stain if a wide area is affected. However, if only a small patch is stained, then it is much more economical to use an aerosol spray sold as stain-block under various brand names. Alternatively, block out the stain with an oil based paint and leave it to dry before applying two coats of emulsion.