Many homes, especially older homes, have at least one layer of oil-based paint on the walls. Even though water-based paint is more widely used today, it does not go on well over an existing layer of older oil-based paint. The oil-based paint often leaves a glossy surface, which makes it difficult, but not impossible, for water-based paint to adhere to the surface. With some preparation, you can paint over oil-based paint with any water-based paint. To learn how to paint over oil-based paint, follow the instructions in this article.
Make sure that the wall to be painted is clean and free of all dust by applying basic detergent and warm water. You can use paper towels to apply the detergent and to wipe away the dirt. Dull the gloss on the oil-based paint by then cleaning the wall with a paint prep cleaner, which will remove the majority of the gloss.
Remove any remaining gloss with sandpaper or an electric sander, if necessary. Use a medium-grit sandpaper and scrub it over the wall to create a surface more prepared for the new paint. If there are any imperfections in the wall such as holes or cracks, fill them with spackle and let them dry, then sand the spackle down flat.
Apply a thick, full coat of wall primer to the wall. Apply the primer by dipping the brush into the primer and shaking off the excess, then painting the primer on the walls. Start with the corners and edges, particularly around doorways and window sills, with the paintbrush. For the bulk of the wall, pour the primer into a large bucket and dip a roller into it, then roll the primer across the wall. Roll from top to bottom until the wall is covered. Let the primer dry completely.
Apply the water-based paint to the wall in the same way that you applied the primer, by painting the corners and edges first with a paintbrush, then by rolling on the paint over the majority of the wall surface. After the preparation steps, the paint should adhere well over the old oil-based paint.
Any oil-based paint applied before 1978 likely contains lead and should not be touched without proper lead removal procedures. Breathing in lead paint dust can lead to serious health problems, so you should never sand a wall that may have been painted with lead paint.