Terms of the Trade: What Is an Undercoat?
An undercoat is essentially a primer for paint — but it serves a slightly different purpose. An undercoat is used to provide an even, smooth surface for a topcoat over existing layers of paint while a primer is used over a new surface. An undercoat will provide thicker coverage than a primer, as well as block moisture from seeping through and fill in any blemishes. Undercoats are especially useful when covering dark shades of paint with lighter ones.