Lap marks appear as a difference in colour when wet and dry layers of paint overlap while painting. To avoid this from happening, maintain a wet edge while painting as a wet painted area remains workable enough to blend. For example, you are starting to paint a section of a wall, but you need to leave the painting for whatever reason. When you return later, the section that you have painted has dried and the wet layer of paint that you are now applying seems to be a different colour. For the first coat this will not be a problem. For the top and final coat, maintain a wet edge in order to be able to blend the entire surface so that the colour dries evenly. 

• Try and paint large surfaces when there is not a strong wind or breeze, as windy areas cause paint to dry quicker.