Until latex paints were developed in the mid-20th century, oil paints were the only game in town, and painters used them both indoors and outdoors. Many painters still prefer oil paints for outdoor applications, but states are increasingly banning them for indoor use. Nevertheless, the wall and trim paint in many older houses is oil-based, and you need to know this before painting with latex, or you may have peeling, chipping and other adhesion problems.
Differences in Flexibility
Latex paint is an improvement over oil-based varieties in many ways. It has lower VOCs -- noxious fumes -- and because it's water soluble, it's easier to clean up. The difference that matters after the paint has dried and cured, though, is the hardness of the paint film. Because latex is rubber, it remains flexible after the solvents have evaporated and the curing process is complete. Oil-based -- or alkyd -- paints, on the other hand, dry to a harder, non-flexible consistency that is more resistant to impacts, which is one of its advantages for outdoor use. Latex paint may not adhere well to oil paint unless you de-gloss and scuff the oil paint first.
Oil Paint Cures Indefinitely
An oil paint finish has a different appearance than a latex paint one, but it usually takes an expert to spot this difference. Oil paint is typically glassier than latex paint, and although modern latex paints formulated with more solids and a stronger binder can rival this appearance, older ones seldom do. One of the reasons for this glassiness is that the oil in oil paint -- which is usually linseed oil -- continues to evaporate for the life of the paint coating, allowing the resins to continue cross-linking to form a progressively smoother surface. That's another reason why coating oil paint with latex is risky -- the latex coating inhibits this process, and the oil paint may crack and peel.
A Simple Test
You don't have to be a paint expert to determine whether your walls are coated with oil-based or latex paint -- a simple test can tell you. Wash a spot on the wall thoroughly with soap and water and let it dry, then moisten a rag with denatured alcohol and rub the paint vigorously. Look at the cloth -- if you see the paint color in it, the paint is latex. If the cloth remains clear, and the paint on the wall is unaffected, the paint is oil-based. This test works equally well for interior and exterior walls, as well as for painted woodwork and trim.
Paint in the Can
It's usually easy to tell if an old can you've been keeping in storage contains latex or oil-based paint. One whiff is all it takes; oil paint has a strong, lingering odor of petroleum or turpentine. Application methods for these two types of paint vary. For example, always use a synthetic-bristle brush with latex paint, because the water in the paint swells natural bristles and makes them go limp. Thin latex paint with water, but use paint thinner, turpentine or mineral spirits to thin oil-based paint. Dispose of both oil-based and latex paint by taking it to your local waste management station for recycling -- never pour paint down the drain or onto the ground.