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Ask the Expert Question-and-Answer Archive by Ron Joseph March, 2007 Painting Plexiglas® (Perspex) Q: I read an archived question you answered in Dec 2005 involving painting plexiglass with latex acrylic and then preserving by sealing with a polyuerathane coating (type used in car finishing). You recommended to wait until the latex loses its sensitivity before applying the polyu coat. How long does it take for the latex to lose the sensitivity? I am working on a church sign project and we would like to paint plexiglass for our signs. Would etching the plexi with sandpaper; applying a Krylon primer spray (the paint used to paint over plastic furntiure, etc); then applying a latex base coat of white then another application of latex with the actual lettering then a final seal of polyu over that be advisable? Thank you for your time and knowlege to assist us. A: Here is a definition of Plexiglas® that I took from the Wikipedia web site: Polymethyl methacrylate (PMMA) or polymethyl-2-methylpropanoate is the synthetic polymer of methyl methacrylate. This thermoplastic and transparent plastic is sold by the tradenames Plexiglas, Perspex, Acrylite, Acrylplast, and Lucite and is commonly called Acrylic glass or simply Acrylic. The material was developed in 1928 in various laboratories and was brought to market in 1933. Because this is a thermoplastic (rather than a thermoset) resin it has a tendency to soften when it comes into contact with certain solvents. The purpose of applying a waterborne acrylic latex paint over the plastic is to try to form a barrier to prevent the solvent from attacking the substrate. One hopes that if a solvent-borne paint is later applied over the waterborne acrylic latex paint, the solvents will not penetrate through to the plastic. My best suggestion is that you take a scrap piece off the plastic, apply the coating system of your choice and determined experimentally if the solvent from the polyurethane with attack the substrate. You should be able to see the results within a few hours, and if you do it observe solvent attack you might conduct additional experiments in which you either apply two coats of acrylic latex followed by the polyurethane, or wait longer for the acrylic to dry and cure before proceeding. Remember, the goal is to prevent solvent from penetrating through the acrylic latex, which in this case since as a barrier coat. Unfortunately, without experimenting with the specific paints and application methods that you will be using I can't give you a guarantee of success. Regards, Ron Joseph
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