WM-02Work out what's behind your wall surfaceTap the wall lightly across its width. Solid brick or block behind plaster gives a dull, solid knock throughout. Plasterboard on timber or metal studs sounds hollow between the uprights, spaced roughly 400mm or 600mm apart, with a firmer knock where a stud sits. Dot-and-dab plasterboard sounds hollow almost everywhere, because the board is stuck to the masonry with adhesive blobs rather than fixed all over, and lath and plaster (common in homes built before about 1930) feels slightly springy and sounds duller and softer. This matters for a mural because a loose or drummy patch on dot-and-dab or lath and plaster won't hold heavy pasted panels. Press firmly across the wall and listen for anywhere that sounds different or moves — get any loose board or plaster re-fixed by a plasterer before you paper over it.