Identifying Oil vs. Latex Paint
I have a question that you can probably answer. When there is an existing painted surface that you want to paint over, how do you determine whether the previous paint is oil or latex? We've had some problems in our house with earlier work painting latex over oil with no primer. We'd like to use latex but we don't want to prime unnecessarily (don't want TOO many coats of paint on a surface). Is there a quick and easy technique to identify oil vs. latex?
I can usually tell whether a paint is oil or latex by the feel of it. Oil paints tend to be very smooth to the touch while latex paints have a more rubbery feel. The difference is more distinct with gloss paints, less so with flat paints. Of course, this method may work for me but is not very helpful if you don't have a tactile "frame of reference" or experience. Since your objective is to paint over the old paint, the easiest way to tell is to rub an area of the paint with either denatured alcohol or a paint deglosser such as Wilbond. If the paint is latex, a small amount of paint may be removed and/or the paint surface will become slightly tacky. If the paint is oil, neither of these things will happen.
The tackiness will normally disappear within an hour. If the paint has a gloss or semi-gloss finish, the gloss will be diminished in the treated area. Paint deglossers are also known as "liquid sandpaper". By removing the gloss (and also cleaning the surface), they allow the next coat of paint to adhere more firmly with no-or-minimal sanding. Oil paints, especially glossy ones, should always be lightly sanded unless they are primed with a tenacious primer such as Bin (shellac), Kilz (oil-based), or Zinnsner 1-2-3 (water-based).
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