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  when doing DIY wall painting is to maintain a “wet edge,” so each stroke of your roller overlaps the previous stroke before the paint can begin to dry.
Thorough preparation is essential to ensure a good finish. Contractors might have to fill cracks or holes and remove old paint if it is badly worn, peeling or flaking. They remove all paint and apply a suitable primer to the bare surface. If the existing paint is in reasonable condition, they clean it and rub it with abrasive material to provide a key for the new paint. On new surfaces, such as bare wood or fresh plaster, they apply a suitable seal, followed by a primer coat.
When their bids are successful, contractors meet customers to finalize their requirements and plan the order and timing of work. Contractors estimate the time required for surface preparation, painting several coats and drying time between coats. For interior painting jobs, they might have to allow time for clearing rooms. Exterior painting schedules might be dependent on the weather in different parts of the country. Exterior painting is not practical in very wet or very cold conditions.

To paint a large section without leaving lap marks, roll the nearly dry roller in different directions along the dry edge, feathering out the paint as you go. After completing the entire length of the wall or ceiling, move to the next section and paint over the feathered edges. For the second coat, apply the paint in the opposite direction. This crisscrossing paint application sharply reduces (if not eliminates) lap marks.

The pros were split on this tip. "Masking tape is problematic," says Mark Dixon, a painter in Missoula, Montana, and author of "House Painting Inside and Out" (Taunton Press, 1997). "Paint can bleed behind the tape, or remove the paint it's stuck to." Another problem is bridging. "Latex paints form a skin," says Dixon. "Removing painted tape can tear the skin, resulting in a ragged rather than a sharp line." Lastly, taping takes time. "Learning how to cut in with a brush takes practice, but if you can do it, you'll leave most tapers in the dust," Dixon says. (Cutting in is painting just the surface you want, not the surface adjacent to it — for example, where a wall meets the ceiling.) On the other hand, "If you can't cut in, you can't beat tape," says Span. The pros we spoke with all recommend painter's (blue) tape because it's easier to remove than masking tape. To prevent bleeding, Span uses a putty knife to bed the tape. After letting the paint dry, he scores the edge of the tape line with a utility knife to avoid tearing the paint.
Buying a can of premium paint, then bait and switch over to low-line products? Again, complete nonsense. Think about it for a moment; the Painter needs 5 gallons of wall finish. So, he buys one can of premium and the rest 'cheap'? How is he going to hide the 4 other gallons? What's he going to hide it in??? He only has ONE gallon of premium. It's not as though he's going to keep older can labels, they would be covered in paint of a different colour.

Colorado Painting Company