SW-06Apply a stabilising primer — method depends on the wallOn solid brick or block with old lime or sand plaster, brush on the stabilising solution generously and work it into the surface — this masonry can take a fairly heavy coat. On lath and plaster, apply it much more sparingly in thin coats over smaller sections; oversoaking can loosen the plaster's key on the laths behind and make it sag or drop. On plasterboard or dot-and-dab with a chalky painted skim (less common, but it happens on old jobs), use a light coat of stabilising primer or a diluted PVA mix (1 part PVA to 5 parts water) instead of a heavy solvent-based product, as solvent can soften the paper face of the board.