To maintain a wet edge, apply your paint towards the unpainted area and then back into the just-painted surface. Brushing from "wet to dry," rather than the other way around will help produce a smooth, uniform appearance. It’s also wise to minimise the area being painted 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.