To waterproof plaster, prime the wall, and apply a heavy-duty exterior paint. If you don't want to use paint, sand the wall, and apply a light coat of linseed oil to the wall. High-quality paint brushes, primer, paint and linseed oil are handy in the project. Be sure to fix leaks often to prevent water from penetrating the walls and damaging the plaster.
Clear the room of items including furniture, cover the floor with drop cloths to protect it from primer and paint, and dust the walls with a large rag. Use a paint brush to apply a thin coat of a heavy-duty primer. Allow the primer to dry.
Once the primer has dried, choose a heavy-duty paint as desired, and use a fresh brush to apply several coats of the paint to walls, starting from the top and progressing toward the bottom. Be sure to apply even coats of paint and not to drop paint on windows and doors. Allow the paint to dry completely before removing drop cloths and replacing furniture.
To waterproof plaster with linseed oil, buff dry the wall, sand it gently, and use a brush with natural bristles to apply a very thin coat of the oil to the wall, working from the top corner and advancing to other parts. Let the oil dry.