A shellac base coat is a layer of resin paint that is applied to a surface to protect the surface from future painting or to create a flat and clean surface to which a finishing coat will stick. Shellac is used as a base coat because its color is resilient when placed on wood finishes, and it can be layered several times to also be used as a finishing coat. Colors and tints can be added to shellac to create a wide range of colors, with white shellac being an effective way to cover up surfaces that have extensive discoloration. The widespread use of shellac as a base coat has declined with the introduction of synthetic coatings.
The substance from which shellac is made is a resin deposited on trees by a type of insect. This resin can take on one of a range of earth-tone colors, from very pale yellows to deep browns. The actual resin contains a small amount of wax that can be useful when applying only shellac from base coat to finish, but that same wax can cause problems with other types of paint that might be applied on top of the shellac base coat.
The wax content of a shellac base coat means only shellac or oil-based paints can be applied over top. Water-based paints and polyurethane paints will have trouble sticking to the waxy surface and could bead up or peel off after a time. A solution to this problem is to use a de-waxed shellac base coat. Without the presence of wax, any paint can be used over top of the shellac.
One type of shellac base coat is called a wash coat. This is similar to a normal base coat in that the shellac is simply applied to a surface, except the shellac is first thinned down with denatured alcohol. The result is a very thin base coat that can help to penetrate and seal a surface that will then receive a finishing coat.
Applying a shellac base coat can sometimes be difficult. Using a brush with shellac can be problematic because, as it dries, brush strokes are left behind. A method that can work without leaving lines is to use a cloth or sponge to quickly get the shellac smoothed on a surface. Other methods, such as using a spray can or spray gun, also can work but are not recommended for small surfaces because of the amount of overspray that can occur.