When the time comes to repaint trim, woodwork or a large surface, it's important to know what kind of material you're painting over. If you plan on repainting with latex paint, you may get premature failure if you apply it over an oil paint, because of a difference in characteristics between the two materials. Painters often use acetone to test whether the material they are painting over is latex, alkyd or polyurethane.
Until latex paint was developed in Canada in the 1940s, most painters used alkyd paint with a base of linseed oil, which is why they still call alkyd paint "oil" paint. The binder in latex paint originally was derived from the rubber tree, but the binders in modern varieties are synthetic products. They may contain acrylic plastic, which hardens the finish, but they still retain the overall flexibility of latex rubber. Latex paint dries to the touch in about two hours, but it undergoes a curing process that takes about three to four weeks to complete.
Need for a Paint Test
Latex is more flexible than the alkyd binder in oil paint, and most painters consider it a mistake to paint latex over oil paint without proper preparation. The flexible latex either lifts easily off the alkyd surface or it causes the harder alkyd paint to crack. Either way, peeling paint is the result. You can't always tell the difference between a latex and an alkyd finish coat by visual inspection, especially on exterior surfaces. One telltale clue is extensive cracking and checking, which is more characteristic of alkyd than latex. In the absence of such cracking, you can use a simple solvent test to settle the issue.
Acetone is a naturally occurring compound found in plants, trees and even in body fat, but most commercial acetone is manufactured. It dissolves a variety of plastics, and is the main component of many brands of nail polish remover. It's useful for testing for latex paint because it softens it, but it isn't a strong enough solvent to soften alkyd paint. You can test a painted surface by moistening a cotton swab or rag with acetone and rubbing the surface. If the swab or rag shows the paint color, it's latex paint. If no color rubs off, the paint is alkyd.
Acetone is a common ingredient in nail polish remover, although not every brand contains it. Check the label and make sure it doesn't say "No Acetone." You can perform the latex paint test with isopropyl alcohol, also called rubbing alcohol, in the event you don't have acetone around the house. It is just as effective, and you may have some in your medicine cabinet. If the test reveals alkyd paint, you can still paint latex over it and get good results if you de-gloss the surface with 150-grit sandpaper or with liquid sandpaper.
- Jupiterimages/Photos.com/Getty Images