On most unpainted exterior surfaces, the standard procedure calls for a primer coat followed by two topcoats of paint. This procedure is also recommended for any painted surface that requires significant scraping and repairs. However, you may be topcoating only because the surface is dull or because you want a new color. If that's the case and the existing paint is sound, a single coat of "one-coat" acrylic latex paint applied properly offers adequate protection and coverage in lieu of the two topcoats.
You can apply quality latex paint over any oil- or latex-painted surface that's in good shape. If the paint is sound, you generally need to prime only scraped or repaired areas. You can also use a stain-blocking primer in lieu of regular primer to seal knots in board siding or trim and to cover stains that you can't remove, such as rust.
If you're applying latex paint over a glossy paint, play it safe and prime the entire house, even if you've sanded or treated the existing finish with a deglosser. Hardboard siding may also require a primer. As a general rule, use an alkyd primer and a latex topcoat when repainting.
Be sure that your primer is appropriate for the surface you're painting. Cedar and redwood, for example, usually require an oil-based primer to seal the surface so that tannin stains don't bleed through the topcoat. Also make sure that the primer and topcoat are compatible. How do you know? By telling your supplier what you're painting, reading the label, and (though not always necessary) using the same brand of primer and topcoat. To make it easier for a colored topcoat to cover primer, have your paint dealer tint the primer to the approximate color of the finish coat.
If you already have three or more coats of oil-based paint on the house, use oil-based house paint. Using latex may cause the old paint to lift off the substrate.
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Source:http://www.dummies.com/how-to/content/priming-your-homes-exterior-before-painting.html
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