A thorough scrubbing is a must before painting any exterior surface. It removes the dirt and broken-down paint residues that keep fresh coats from adhering and gets rid of mildew that grows on paint in all but the most arid climates. Most contractors clean with pressure washers, but in the hands of someone unfamiliar with the equipment, these can gouge wood, shatter glass, and drive water behind siding and trim. Using a hose, a pump sprayer, and a scrub brush is slower but safer, and just as effective.
A true professional understands that it is his or her responsibility to provide a quality job for the homeowner. Professionals also know that attention to detail is a part of the job. While a homeowner might leave a bit of paint on the window pane, a dedicated interior painter for hire will make sure that these small details are handled properly, because a perfect job is the best calling card.