Use Extenders to Avoid Do-Overs
The longer that paint stays wet on walls or woodwork, the fewer lap marks and runs you'll have to deal with. Lap marks are those dark, ugly lines caused by painting over an area that's already dry. If you can keep the paint wet longer, you won't have to worry about them. And the way to prolong the 'open time' of paint is to add a paint extender or a conditioner (Floetrol is one brand; at home centers and paint stores).
Pour all the paint you need for the room into a 5-gallon bucket, then stir in the extender or conditioner (following the manufacturer's recommendations). Because extenders and conditioners help level out brush marks and paint runs, you won't have to go back and fix them later.
Cut in One Wall at a Time
Once you have your paintbrush in hand, it's tempting to cut in along all the trim, the ceiling and the corners in the room. But you'll get better results if you cut in just one wall, then immediately roll out the wall before cutting in the next one. That's because if you roll out the wall right away, while the cut-in paint is still wet, the cut-in paint and the wall paint will blend much better, reducing the chance of lap marks.
Spend Less Time Taping
You probably know that taping off all your trim with masking tape is time consuming and doesn't guarantee good results—paint can still bleed under the tape. In short, taping off everything is a waste of time. Instead, only tape horizontal surfaces, like baseboards and chair rail, where paint splatter can land and be noticeable. Vertical surfaces, like door and window trim, aren't as vulnerable to splatter, so don't bother taping them. Just be sure to cut in carefully with your paintbrush so you don't slop paint onto the trim.
'Tape creates its own set of problems, like coming off before you start painting and pulling paint off the wall when you remove it,' Bill says. 'But I learned the hard way that you still want to tape baseboards—paint will always splatter on baseboards if they're not taped.'