Applying latex paint over oil-based paint follows the same basic steps as any repainting job: scrape, sand, prime, and paint. Preparation and priming are two crucial, but sometimes skipped, steps required for a quality job that will last a long time. In this case, a coat of primer is essential to enable the new finish to bond with the hard, non-porous, oil-based paint. (In addition, the primer will hide stains and create the ideal substrate for the new paint to level out and cure to a smooth finish.)
But how do you know if an existing finish is oil-based or not? A simple test can determine if the finish is oil-based. Take a rag saturated with denatured alcohol and rub the surface. If you have an oil-based finish, the solvent will do little more than clean the surface. If the coating is latex or water-based, the solvent will soften the paint finish.
The possible presence of lead, most frequently found in paint applied prior to 1978, is another issue to consider. In this case, use a lead-paint testing kit, available at most paint or hardware stores. If you find the presence of lead, Before you start, find out how to protect yourself and your family by contacting the National Lead Information Hotline at 1-800-424-LEAD or log on to http://www.epa.gov/lead. For work that does not contain lead–whether it the old coat is oil or acrylic–follow good painting prep practices to ensure a satisfying outcome: Scrape away the loose, flaking old paint, sand the surface until the glossy areas are dull and smooth so there is no perceptible transition between the old paint’s layers. Use a sanding block or sandpaper; liquid deglossers sometimes leave behind a residue. Then, apply a primer like Benjamin Moore’s Fresh Start High-Hiding All Purpose Primer (046). After an hour or two of drying, the surface will be ready to repaint with a fresh coat of acrylic paint.