Lapping is the appearance of a denser colour or increased gloss where wet and dry layers overlap during paint application.
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 advised to work in manageable-size areas and plan for interruptions at a natural break, such as a window, door or corner. Using a top quality 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, reducing wet edge time and therefore making lapped areas noticeable. Solvent-based paints generally have superior wet edge properties, and therefore less lapping.