Lap marks appear as a denser colour or higher gloss where wet and dry layers overlap.
What Causes It?
Lap marks can occur when wet and dry layers overlap during painting or staining. They can also occur when paint is applied in direct sunlight or in conditions that are too warm, or when paint or stain is applied on too porous a surface.
How to Solve It
To cover lap marks, spread another coat of paint on top of your previous coat, taking care to spread it uniformly. If the finish is transparent or the surface is overly porous, you may need to apply a second coat or a primer coat.
To avoid making lap marks, do not paint sections of large surfaces from top to bottom completely. Instead, paint walls and other surfaces in sections small enough to allow you to maintain a wet edge.
Avoid painting on hot, windy days, as this will accelerate the drying time of paint.
Note: Images provided by The Rohm & Hass Paint Quality Institute.