Lapping:

Appearance of a denser colour or increased gloss where wet and dry layers overlap during painting.

Possible causes:

• Failure to maintain a “wet edge” when painting.
• Use of a low solids “economy” paint.

• Maintain a wet edge when painting by applying paint toward the unpainted area and then back into the just painted surface. This technique (brushing or rolling from “wet to dry” rather than vice versa) will produce a smooth uniform appearance. It is also wise to work in manageable size areas; plan for interruptions at a natural break, such as a window, door or corner.
• Using a top quality acrylic water-based paint makes it easier to avoid lapping problems because higher solids (pigments and binder) content makes lapped areas less noticeable.
• If substrate is very porous, it may need a primer/sealer to prevent paint from drying too quickly and reducing wet edge time. Solvent-based paints generally have superior wet edge properties.