Painting Plastic

Ask an Expert From: Products Finishing, ,

Posted on: 1/11/2011

One of my good customers brought in a plastic engine cowling from a 30-year-old radio controlled aircraft kit that he had trouble painting. He said he thought the cowling was ABS and has been building this model for the past 6 years! He was unable to keep the primer and paint from peeling off the ABS plastic. When he pulled off the masking tape, the paint and primer came off with it.

Q1. I run a job shop painting company. One of my good customers brought in a plastic engine cowling from a 30-year-old radio controlled aircraft kit that he had trouble painting. He said he thought the cowling was ABS and has been building this model for the past 6 years! He was unable to keep the primer and paint from peeling off the ABS plastic. When he pulled off the masking tape, the paint and primer came off with it. He had to use acetone to remove all of it, as it has a dummy radial engine molded into the cowling, which made it hard to sand.


When he brought the part over to my shop, he asked me to paint it using a fuel-resistant primer and paint, which provided a beautiful finish until I removed the masking tape. Then I had to spend a lot of time stripping the paint, sanding and washing with isopropyl alcohol. I prepped as much of it as I could get at by sanding with 220 grit paper.


Next, I tried spraying a light coat of primer, letting it dry for 24 hours, sanding with 360 grit paper in preparation for a second light coat of primer, but when tested with masking tape it peeled again. I am at a complete loss as to what I should try next. Can you advise me? T.P.

 

A1. Are you sure the plastic aircraft kit cowling is ABS? I question this because acetone would have attacked the ABS while your customer was cleaning paint and primer off its surface.


The recommended procedure for preparing ABS and other plastic substrates for painting are similar to preparing any other substrates. The substrate must be clean, free from oily soils, release agents and particulates. To remove oily soils, plastics are cleaned using alkaline cleaners, detergent cleaners, emulsion cleaners and compatible solvents. Release agents are best removed using recommendations from their suppliers.


Certain plastic substrates require special pretreatments to promote adhesion after cleaning. If necessary, these plastic substrates are pretreated using abrasives, chemical etching, corona discharge, combustion spray, plasma spray, and lasers. When sanding is used to abrade the substrate, the grit must be fine enough to prevent scratch or gouge marks on the surface that will show through the paint.


And as always, the applied coating must be compatible with the substrate. Compatibility of the coating with the substrate means its solvents should not attack, dissolve or otherwise destroy the substrate. Although, it is possible in some cases that a slight solvent attack can promote adhesion. Other plastic substrates must be primed to promote adhesion or to prevent attack by the solvents in a topcoat. If you are positive the plastic cowling is ABS, and I really doubt it, go to your local auto parts store and buy a can of primer specifically formulated for ABS.



Q2. Thank you, Carl, for your good advice. My customer thinks for sure it is ABS plastic, but maybe not. It’s a fairly lightweight opaque plastic that doesn’t like lacquer thinner, but is fine with acetone. I picked up a spray can of adhesion promoter at the auto parts store which is supposed to stick to any surface. I’ll test it on some scrap plastic and let you know how it works. T.P.

 

A2. There is a fairly new product in spray cans formulated for painting plastics. If you are really painting ABS, it should work. Again, there are some plastics that are practically unpaintable and others that require special pretreatments that are unavailable outside industrial settings. In retrospect, the acetone didn’t attack the plastic because it evaporated so quickly. On the other hand, the lacquer thinner contains slower evaporating solvents and stays “wet” long enough to attack the plastic.



Q3. I considered using the adhesion promoter but decided against it. Instead, I switched primers. I am now using an automotive light gray primer-surfacer that I’ve used in the past on all my fiberglass parts. It has always worked well and so far seems to be sticking to the plastic. I haven’t applied the topcoat yet, but I tested the primer-surfacer with the masking tape pull and nothing peeled off, where as with the fuel resistant primer it did. T.P.

 

A3. Sounds like you are on the right track, T.P. If the primer-surfacer adheres to the plastic, you can use any top coat over it. Speaking of “on the right track,” you said your customer has been building this aircraft for six years. That’s not too bad. I’ve been building my model railroad for twenty years. By the way, you are invited to the Golden Spike ceremony, on July 13, 2032. At the rate I’m going, it will take me that long to finish it.


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