Now take your 100mm trowel and apply a fresh batch of compound thinly across the joint. Spread it slightly wider than the joint so it blends in smoothly with the surface of the wall. Once it has dried, the joint is ready to have a waterproof membrane or tiles applied over the top. If you plan to paint the surface, apply a third, slightly wider coat of compound first. To finish off, fill in any nail holes with a small amount of compound.