Do You Need to Undercoat Walls?

Paints and Primers

• Enamel is varnish or resin based, and is not a true paint. It always needs an undercoat unless the surface has an existing finish. All finishes and surfaces must be sanded before applying enamel to prevent peeling. Latex, or water-based, primers can't be used beneath oil-based or enamel paint. However, oil-based primers can be used under both latex and oil-based paints. Oil-based primers are not only used to seal wall surfaces, but to cover stains and to prime a dark wall for repainting with a lighter color.

Plaster and Drywall

• Unpainted plaster requires two coats of oil-based primer before finishing with either a latex or oil paint. Unpainted drywall finished with drywall mud requires oil-based primer. Previously painted plaster and drywall patched with drywall mud require oil-based primer. Latex primer soaks into the mud, leaving uneven coloring in the finish coat.

Wood Paneling

• Always use oil-based primers on wood paneling. Latex primers can raise the grain of the wood due to their water content. Clean the paneling thoroughly and sand it lightly before applying the primer to help it stick to the surface. Two coats of primer may be required to seal and prepare the wood for painting. After the primer has dried completely, sand the surface lightly before painting to avoid future peeling.

Painting Vinyl

• Painting vinyl walls or wallpaper requires an undercoat of a special vinyl primer. These acid-based primers etch themselves into the surface to prevent peeling. Two coats is recommended, with each coat sanded lightly with an extra-fine-grained sandpaper before applying the next primer coat or the finishing coat of paint.