Sanding not only feathers out chipped paint but also provides "tooth" for the next coat of paint. For glossy trim, use a sanding sponge rather than sandpaper. Sponges mold to the shape of the trim and last longer than paper. When applying latex over alkyd paint or when he is not sure of the original finish, Brian Doherty, a painter in Richmond, Virginia, follows the hand-sanding with liquid sandpaper to make sure the surface is completely deglossed to prevent incompatibility problems. "I've seen homes where latex was used on oil-painted trim, and the paint started to peel in less than a year," says Doherty.
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.
You are right on with this - why do people leave switch plates on when it's just so easy to take them off? Another thing that happened to us - we had the popcorn ceiling taken off and the ceiling painted white. When the job was done and I later went to change out all the fixtures/fans, they had left every fixture in place, so there was a large patch of popcorn and unpainted ceiling left behind - it just didn't dawn on me to specify that they take those down before scraping and painting. It was kind of a mess.