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Priming
For unpainted exterior surfaces it is necessary to apply one coat of primer followed by two coats of topcoat. For any painted surface that requires significant scraping and repairs, you should also follow this procedure. If the existing paint is sound, a single coat of one coat acrylic latex paint applied properly offers adequate coverage. You can apply the latex paint over any oil or latex painted surfaces that you repair. It can be useful to sue a stain blocking primer instead of regular primer to seal knots in board or trim in and to cover stains that you can’t remove such as rust. For rusty nail heads you will need to sue a stain blocking coating such as rust inhibiting metal primer to ensure the rust does not bleed through your paint job. You will first need to scrape off all the caulk and glazing compound around the nail head and set any nail heads that are exposed.
If you’re applying latex paint over a glossy paint, play it safe and primer the entire hose even if you’re sanded or treated the existing finish with a de-glosser. In general, use an alkyd primer and a latex topcoat when repainting.
Be sure the primer is suitable for the surface you are repainting. For example some wood siding such as cedar and redwood usually require an oil-based primer to seal the surface so that tannin stains don’t bleed through the topcoat. It is also important to check if the primer and topcoat are comparable. Your paint supplier will be able to tell you this or you can check the label. You can also make it easier for a colored topcoat to cover primer by asking the paint dealer to tint the primer to the approximately color of the finished coat.
Remember that if you have already uses three or more coats of oil-based paint on the house, use oil based house paint. Using latex paint may cause the old paint to lift off the substrate.
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