#FastTipTuesday - Avoid Marks: Keep It Wet!
Lap marks occur when you roll over paint that's already partly dry - creating stripes.
To avoid lap marks maintain a 'wet edge', so each stroke of your roller overlaps the previous stroke before the paint begins to dry. Start near a corner and run the roller up and down the full height of the wall, moving over slightly with each stroke.
Don't let the roller become nearly dry, reload it often so that it's always at least half loaded.