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Shop Tip – Flame Treating Plastic to Make the Paint Stick February 3, 2010

Posted by Deb Kosiba in Shop Tip.
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Yesterday I needed to paint a plastic part, and I knew from experience that it was the kind of plastic that the paint would not stick to.  So I used a technique I learned a few years ago that helps the paint stick.  While I was doing this, it occurred to me that I should share this trick with all of you.

Some plastics take paint just fine but some don’t.  If the plastic is flexible, the paint will often chip and flake.  But even some rigid plastics will have problems, sometimes with pitting or orange-peeling.  This happens most often with plastic made from polyethylene and polypropylene.

There are some common solutions, such as scuffing it with sandpaper, or rubbing it down with a mild solvent.  The first will give the plastic some “tooth” for the paint to grab on to, the second will remove any oils on the surface that might keep the paint from sticking.  But neither is a perfect solution.  The piece you sanded is left with a texture that may show through your paint job, and the solvent doesn’t always solve the problem.

Professional companies prepare plastic parts with something called a Corona Treatment, which sort of zaps the surface with an electrical discharge. The discharge changes the surface enough that paint and ink will stick to it. But a Corona Treatment takes a lot of expensive equipment.

A cheap way you can get similar result at home is to Flame Treat the surface.

Basically you take a flame, such as a propane torch, lighter, or match, and you barely lick the very tip of the flame over the surface of the plastic. Some web sites even suggest that you use the area of the flame that is the gassy fumes just past the tip.  You need to move quickly, the sorts of plastic that respond well to the flame treatment also melt quickly.

On larger items, you can tell when the treatment has worked, the plastic loses it’s gloss and now has a matte finish.  On tiny pieces, you can’t really tell, you usually just have to trust that it worked.  I can’t tell you how many times I’ve treated a piece multiple times because I wasn’t sure, only to end up melting it.

With a bit of practice, you can get pretty good at it, and you will learn what the plastic looks like when it works.  I would suggest trying it out on some scrap pieces before doing this procedure on something expensive or irreplaceable.

And remember, safety first! Flame treating plastic involves fire, so take all the necessary precautions when dealing with an open flame.

If you have tried this technique out on one of your 3-D SF&F projects, let me know how it worked out for you!

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