Even the best exterior paint can fail if it’s applied incorrectly. Always use a primer when painting any untreated surface to seal it off and to provide a base for topcoats to stick to. Alkyd primers are best for bare wood because they cover bleed-through from wood knots better; be sure the label states that the primer is designed to stop bleed-through. Water-based primers are a good choice if knots aren’t an issue. Water-based paints are compatible with both types of primer.
When repainting, prime only when necessary. If the paint hasn’t cracked or flaked, you may not need to prime at all. Advantage 900 from Porter Paints is designed to go over any existing paint without priming or sanding. If you must scrape down to bare wood, spot-prime. Not sure whether priming is needed? Try this test: Paint a small portion of the wall and let it dry. Then put an adhesive bandage on the newly painted surface and snap it off. If paint sticks to it, the old paint won’t support a new coat and requires a coat of primer. If the bandage is clean, power wash the siding and paint.
For painting new construction, the FPL recommends dipping each piece of siding in a paintable water repellent, priming, then applying two coats of water-based paint. The lab also suggests installing siding on furring strips, creating a ventilated space behind it to reduce vapor. Seal the bottom with screening to keep insects out.
If you hire a painting contractor, be sure he or she follows the paint manufacturer’s directions. For example, the temperature should be between 50° and 90°F to apply water-based paint. Also, the topcoat should go on within two weeks of the primer. If you wait too long, the mechanical bond between the two won’t be as strong because the surface texture of the primer breaks down. And if two topcoats are used (recommended for new construction), the second should go on within two weeks of the first.
There are lots of exterior paints out there. Knowing how to pick the best from a lineup of look-alike cans will help your paint job last well into the next century. To see more on this copy and paste or click on the link—>http://www.thisoldhouse.com/toh/article/0,,219538-4,00.html