CAUSES: Paint usually peels from window sills and frames because of condensation. Water standing on the sills eventually penetrates the paint film. When it later evaporates, the pressure from under the film causes peeling.
SOLUTIONS: Remove all the paint by scraping, sanding and, if necessary, using a chemical stripper. The use of a heat gun is a good alternative to chemical strippers. Either method can damage unprotected glass or window-sealing materials. Caulk where necessary especially at every 90 degree angle and crevice to eliminate any opportunity for moisture to penetrate into the wood substrate. Coat the sill with an exterior primer and finish coat. When painting frames, leave a small 1/16 inch edge of paint on the pane to prevent water from getting behind the film. Open vent holes in aluminum window trim.
Other solutions involve using cedar wood as a replacement for pine, which soaks up water like a sponge. Paint the butt ends of the wood before replacing the sills.
Use oil-based gloss paints for surfaces exposed to standing snow and rain to help keep water from entering the wood. On new wood, use two topcoats of oil-based gloss. (Check safety and environmental regulations and codes.) Some manufacturers have latex primers which are impervious to water.
The use of storm windows and dehumidifiers helps prevent recurrence of peeling.
