To waterproof plaster walls, apply prime and paint to prevent water from penetrating the surface and damaging the plaster. For areas where color is not desired, apply linseed oil to the entire wall.
- Prime the wall
Protect the floor and surrounding areas with drop cloths and use painter's tape to protect around doors, windows and trim. Use a roller to apply high-quality primer to the wall in broad, even strokes. One coat of primer provides a smooth foundation for paint to adhere, offering consistent color throughout and preventing the wall from absorbing too much paint.
- Paint the wall
Use a roller to apply high-quality paint to the wall. Painting the wall is just like priming it. Use long, steady strokes to get an even color of paint. It may be necessary to apply two or three coats of paint to get full coverage, depending on the quality of paint and the paint color. Once dry, remove the painter's tape and drop cloths from the space.
- Apply linseed oil
For spaces where color is not desired, use linseed oil to waterproof the plaster. Use a paintbrush with natural bristles to apply a light layer of linseed oil to the wall, working from top to bottom.