Before you paint clean and dry the surface thoroughly, removing all dirt, dust, rust and grease. On bare wood, coat knots and sappy areas with stain block primer/sealer. Then prime entire surface with exterior oil or latex primer. On previously painted surfaces, scrape, sand, then prime. Wire brush badly chalking areas. Dull glossy surfaces with gloss remover or sandpaper. Countersink nails and fill with putty. Caulk all cracks and joints.
On unprimed, pre-primed or previously painted hardboard siding, prime with an exterior oil primer.
On masonry, wire brush loose materials from chalky areas.
On metal, remove rust on galvanized gutters and prime with a rust inhibitive primer.
On new galvanized surfaces, allow to age 6 months before painting or clean with muriatic acid.
On unpainted metal surfaces, remove rust, clean with mineral spirits and prime with a rust inhibitive primer.
On vinyl siding, wash weathered vinyl siding thoroughly and allow to dry. Then paint with a color lighter than the original siding color.
WARNING! Painting vinyl siding with darker colors will cause warping or deformation from heat absorption.
CLEAN MILDEW FROM THE SURFACE! Mildew is a fungus that looks like dirt, but won't wash off. Mildew must be removed before painting or it will grow through any new coating. To remove mildew, scrub with a commercial mildew remover or a solution of 1 part liquid chlorine bleach to 3 parts water. Rinse thoroughly and allow to dry. Protect eyes and skin be wearing protective equipment, including rubber gloves and eye goggles when working with the bleach solution.