A fresh layer of white paint can brighten up a room, staircase, door, porch or piece of furniture instantly. The goal is to keep the paint at its whitest for months and even years to come. However, white paint often yellows over time. In order to keep white paint from yellowing, purchase the right kind of paint and seal it with a top layer which will prevent yellowing.
Use water-based white paint, which doesn't yellow as oil-based white paint does. Oil-based paint yellows especially when little natural light is present. Alkyd and oil-based paints contain a curing mechanism which cause them to yellow when not exposed to sunlight. Paint also yellows from heat emitted from stoves and radiators, from lack of light such as behind pictures, appliances or inside closets, from ammonia, tobacco and other contaminants and from excessive moisture.
Add on a layer of a white latex or acrylic paint to reduce the amount of yellowing to the already yellowed paint. To keep the white fresh, however, avoid the environmental conditions which caused it to yellow in the first place.
Use a primer or sealant on top of the yellowed paint instead of the latex paint or instead of repainting with a fresh layer. After applying the primer, you may choose to apply a fresh layer of water-based paint, and then add a lacquer or varnish on top as a finish.
For interiors, use wall and wood interior latex paint, an interior oil-based primer, or an oil primer or undercoater. Use varnish or other types of finish as an optional step in keeping the paint white longer.
For exteriors, use an exterior primer or an all-surface enamel primer. Use a varnish for exteriors for longer duration of the white paint.
Keep sunlight as much as possible on the white painted areas to keep the paint from yellowing. Avoid smoking, using a grill, cooking or baking if possible in the areas with the white paint to prevent yellowing. Or use a primer, finish or sealant in these areas to help keep the white longer.