If you have oil-based paint on your walls but want to give them a makeover with latex paint, prepare the walls properly. Failure to do so can result in the latex paint not adhering properly. It's not as simple as just slapping on a coat of latex paint and calling it a day. Latex paint will chip right off if it's applied directly on top of the oil-based paint. Take the proper steps before applying the paint and the finished product will be well worth your time.
Mix 2 tbsp. of trisodium phosphate (TSP) cleanser and 1 gallon of warm water in a bucket. Fill another bucket with plain water.
Dip a sponge into the bucket of cleaning solution. Scrub the walls, starting at the top and working your way down, with the sponge. Work in sections of no more than about 4 square feet at a time so the cleaning solution doesn't dry onto the walls. Rinse each section with a clean sponge and plain water. Let the walls dry completely before proceeding.
Sand the entire area with a 220-grit sanding sponge. This will dull the oil-based paint and provide a better surface for the primer to adhere to. After sanding, remove the sanding dust with a soft brush hose attachment from your vacuum cleaner. Wipe down the walls with a microfiber tack rag to remove any remaining residue.
Fill a paint tray with a shellac-based primer. Use a roller to spread a thin, even coat of the primer over the oil-based paint. Apply the primer to corners and trim areas with a small paint brush.
Let the primer dry completely according to the manufacturer's instructions. Sand the entire area to be painted once again with a 220-grit sanding sponge to roughen up the surface, to help the paint stick. Wash down the walls again with a fresh mixture of the trisodium phosphate solution. Rinse the walls with clean water and let them dry completely before applying the latex paint.