Lap Marks are the appearance of a denser colour or an increased gloss where wet and dry layers overlap during paint application.
What Causes It?
Failing to maintain a "wet edge" when applying paint or using low-solid, "economy" paint can both lead to the formation of lap marks.
How to Solve It
When painting, make sure to maintain a wet edge 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 manageably sized areas, and to plan for interruptions at a natural break, such as a window, door, or corner.
Using a top-quality acrylic latex paint makes it easier to avoid lapping problems because higher solids (pigments and binder) content makes lapped areas more noticeable.
If the substrate is very porous, you should apply a primer or sealer to prevent paint from drying too quickly and reducing wet-edge time. Alkyd paints generally have superior wet-edge properties.
Note: Images provided by The Rohm & Hass Paint Quality Institute.