Maintaining a “wet edge” during the painting process is very important. This means that the just-painted areas must be still wet when you apply the next stroke of the brush or roller. If the edge dries up, the overlapping edges will be seen when the paint dries. Using poor quality paints that are high in solid pigments also result in lap marks. This problem can also be caused if you are painting a porous or warm wall that makes the paint dry quickly.
Always keep a wet edge while painting. Go back to the just-painted area before it dries and then continue to the newer area to maintain uniformity. If you have to take a break, choose an area of manageable proportion such as a door or a window. Don’t stop painting a wall halfway. If the wall is porous, apply a good sealer or primer so that the surface becomes smooth and the paint does not dry up quickly. Use a good quality low solid acrylic latex paint as it does not cause lapping problems. Alkyd paints take time to dry and therefore do not have lapping problems.