Oil-based paint once ruled the world of interior paint and coatings, particularly in high-impact areas such as trim work, doors, and cabinets. Yet with the nearly total phase-out of oil-based paint in the U.S. in 2005, the question of whether you can paint latex over oil-based paint is even more confusing. If a house is old enough, it is inevitable that it will have oil-based paint somewhere; latex paint wasn't introduced until the 1940s. And newer houses might have oil-based paint, as it is not entirely banned. It is still available in quart sizes or smaller, and many professional painters favor oil over latex for a smoother, rock-hard finish.
Latex paint can be applied over oil-based paint. As long as the surface is fully cured, there is nothing inherent in the oiled-based paint coating that precludes an additional layer of latex paint. Preparation is all-important. Glossy surfaces will not take a second layer of paint well, so they need cleaning, sanding, and more cleaning with a damp cloth or a tack cloth. Use a primer before putting down the latex. While only advisable for many painting projects, a primer is a requirement for this one. If you have multiple layers of oil-based paint, consider scraping off the paint first.
Though latex can go over oil-based paint, the opposite is not recommended: oil-based paint should not be applied over latex paint.
Surfaces Typically Coated in Oil-Based Paint
There is a very good reason for using oil-based paint. Oil-based paint is considered to be self-leveling. After the brush delivers the paint, the paint levels flat, leaving zero brush marks, gaps, bubbles, or holes. Door frames or window sills painted in an oil-based paint can be a beauty to behold, a museum piece, glass-smooth and rock-hard.
Key surfaces in the home that might have oil-based paint include door casing, window trim, baseboards, crown molding, fireplace mantels, and kitchen cabinets. Since these are high-impact surfaces subject to wear and degradation, there will come a time when you need to paint them again.
One reason why oil-based paint is no longer in favor: volatile organic content (VOC). When it dries, oil-based paint emits a host of VOCs that are harmful to the environment. On an individual level, homeowners often find oil-based paint's clean up procedures messy and smelly, since a spirit must be used, not water.
How to Paint Latex Over Oil-Based Paint
Applying latex paint over oil-based paint has less to do with the lower coat being oil-based than with the fact that this is a paint-over-paint project. Even when using similar types of paint, it is difficult to achieve the same adhesion as you would by laying down paint on primed wood. As a result, this project is mostly about preparation. Once the surface is adequately prepared, it is a simple matter to paint the material.
Scuff the Surface to Remove Gloss
With fine grit sandpaper, scuff down the surface of the oil-based paint layer for improved stickability. Aim for eliminating surface sheen, or gloss. The gloss is as much an enemy of paint adhesion as is dirt. Pry off any loose paint with a five-in-one tool or putty knife. Fill small gaps and holes with wood putty or wood filler.
Damp Clean to Remove Debris
Many layers of dust and dirt may compromise latex paint's ability to stick. While some latex-over-latex projects may allow you to bypass cleaning, this project is different and requires deep cleaning. You will need to build drying time into the project if using a damp cloth.
Add easily available and inexpensive, TSP (trisodium phosphate) should be added to the water to improve cleanability. TSP is available in most hardware and home improvement stores and easily mixes with tepid or warm water. Let the damp surface thoroughly clean.
Dry Clean to Remove Last Specks of Dust
Use a tack cloth to lightly wipe down the surface. Tack cloth is an inexpensive, simple product made of cheesecloth impregnated with beeswax. Its sticky surface is perfect for vacuuming up those last stray motes of dust. Do not rub too hard. Applying hard pressure will result in leaving wax on the surface, and this can be harder to remove than dirt.
Paint the Surface
With the surface completely dry and clean of dirt, you can now paint it as you would any other surface. Use a high-quality paint brush for best results.