It is often necessary to know what a particular coating is before you paint over it, so it can be prepped correctly. Is it oil or latex ? Here are a couple of easy tests you can do to figure it out.
You can test a painted surface by moistening a rag with acetone and rubbing the surface. If the rag shows the paint color, it’s latex paint. If no color rubs off, the paint is alkyd. Acetone is commonly found in nail polish remover. Check the label, some contain no acetone.
You can perform the latex paint test with isopropyl alcohol, (rubbing alcohol), if you don’t have acetone. It is just as effective, and you may have some in your medicine cabinet.
A lesser known test is the lye test. A solution of two tablespoons lye to a cup of water applied to a painted surface will cause the paint to bubble and blister if it is oil base paint. Latex paint will be unaffected. This is a good test to use if you have paint chips. Drop a couple into a jar with this mixture. The latex paint chips will not be changed, the oil base chips will get eaten up.
Remember to take caution, It’s lye!!