So what are lap marks? Lap marks are those ugly stripes caused by uneven layers of paint buildup. They occur when you roll over paint that’s already partly dry. (In warm, dry conditions, latex paint can begin to stiffen in less than a minute!) The key to avoiding lap marks is to maintain a “wet edge,” so each stroke of your roller overlaps the previous stroke before the paint can begin to dry. Let us know your tips in the comments below.