As any painter will tell you, prep work is just as important -- if not more important -- than the paint application itself. If the paint on your house is chipping or peeling, it needs to be scraped and sanded smooth before applying a new finish. In most cases, you won’t need to remove all the existing paint, but scraping, sanding, patching, caulking, and priming are critical steps that take place before the start of most exterior painting jobs.
The house bath may reveal nails that have popped out of the siding or rusting nail heads that have left streaks of rust on exterior walls. If so, use sandpaper or steel wool to clean the nail heads. On clapboard siding, use a nail set to recess the nail head about ⅛ inch below the surface of the wood. Dab on a coat of rust-inhibiting primer (unless the nail is aluminum or nonrusting galvanized steel), and let it dry. Then fill the nail hole with spackle or putty. When the filler is dry, give it a coat of primer. For flathead nails, which cannot be recessed, sand the heads until they're shiny, and coat with primer.
I made the HUGE mistake of hiring Certa Pro to do several interior rooms of my house, and remove popcorn ceiling in a bathroom. What a nightmare! They didn't paint any door jams, they broke a cedar window sill-and didn't bother telling me, they gouged a hardwood floor, they never sealed the room that had the popcorn removed--causing white powder to be in all rooms of a 2 story home. I can go on and on. Horrible company
Depending on just how dirty the outside of your house is and on the house's size, there are two ways to approach this job. If you live in an average-size house, use a garden hose with a carwash brush attachment to bathe the big areas. For caked-on dirt, use a scrub brush or a sponge and a pail of warm water with a good, strong household detergent in it. Work from the top down, and rinse all areas where you scrubbed with water.