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.
If less than half the old paint is left, however, it may be worth stripping it all off. Guertin gets rid of stubborn remnants using shrouded grinders (like the PaintShaver), infrared paint strippers (such as the Speedheater), or chemical strippers (like Multi-Strip), then smooths the wood with a course or two of sanding. When siding (or bank accounts) can't take the shock of a total strip job, Rich O'Neil, of Masterwork Painting in Bedford, Massachusetts, has successfully hidden rough, well-adhered paint under Peel Bond, a thick primer.
My painter franchisee, who I got from Angie's List, must have found some shmucks on the corner to paint my house. I specifically said I wanted stain. First my house was painted the wrong color, Then it was painted with LATEX paint- not stain. Estimates to 'undo' the damage is around $10,000 and its not a guarantee that it will work. The work is at best amateurish. Many underpainted or not painted spots, drips, etc. They did the entire outside (scraping?, priming? caulking?, painting), cleaned up and left within the time I left for work and came home. There were thee of them (my neighbor told me) This was two years ago and I am still upset. Can't be fixed.