It is important to differentiate the type of paint that is currently on your building before you put on another coat of paint. If you use the wrong type of paint, you could find it peeling a week or so later. You can also look at this website to hire a professional wall paint tester online.
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Oil and Water Based
You may remember from your high school chemistry class, that oil and water do not mix. All paints and stains are either oil or water-based and when they are combined, the results can be disastrous.
More specifically, water-based paints cannot stick to the oil-based product, thus it runs or peels. Since water paints sit on the top of the surface, the oil-based products are not able to penetrate the wood and never set.
The Transparency Test
Most oil-based paints are designed to sink into the wood. Stains and varnishes can easily be detected by examining the surface of the building. If you can see the wood grain and the stain simply colors and enhances the wood, it is an oil-based product being used.
However, more and more non-transparent stains are being used. These stains still sink into the wood, but they give the appearance of paint by covering all aspects of the surface. For this reason, the transparency test can be trusted to determine transparent oil-based products, but should not be the only method for determining solid or semi-transparent paints.
The Goof Off Test
Goof Off and similar products remove all types of things, such as grease, markers, and latex (water-based) paints. Buy a bottle of goof off at your local hardware store.