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Stuck On Plastics: Adhesion Problems

Ever had a flaky experience when refinishing plastic parts? You’re not alone. To help prevent further adhesion problems, consider these common-sense cures — used by a pro whose history with plastics is long and sordid.

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Writer Mike West, a contributing editor to BodyShop Business, retired and closed his shop in Seattle, Wash., on July 1, 2011, after more than 40 years in collision repair. The mayor of Tukwila declared that day “Mike West Day” to honor West for his 39 years as a businessman in the city. He is keeping active by restoring a 1933 Rockne Sedan Delivery and a 1934 Pierce-Arrow Rumble Seat Coupe. He plans to continue administering the I-CAR in-shop welding series in the Western Washington area.

So much topcoat, undercoat and every other kind of coat has flaked off so many plastic parts after being applied by collision repairers that it’s become a troublesome issue. So troublesome that it’s doubtful there’s a refinish technician out there who hasn’t experienced some kind of adhesion problem on plastic parts.

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And the problems come in all sizes and varieties. The many different types of plastics needed for different uses and different levels of flexibility create one set of problems. Another set of problems: Some parts are purchased already primed or painted, while others are in raw plastic condition. And when it comes to achieving a color match on plastic parts, additional work — compared to what’s needed on sheet-metal parts — may be required. Adhesion can also be tricky.

How do you avoid such problems when refinishing plastic parts? A clear understanding of the plastics you’re refinishing, as well as their individual requirements, will help ensure success.

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Plastics and I Go Way Back
I began working on collision-damaged vehicles long before we began to paint plastic parts that are common today. Of course, plastic parts were used by auto manufacturers prior to the creation of common refinishing processes for repairers.

The first plastic painting problems I remember were on interior trim panels, when the OEMs switched from fabric- or vinyl-covered interior components to colored, single-component plastic trim parts. Door-trim panels, kick panels, dash pieces, sidewall interior trim, etc., were some of the first plastic parts to be used in interiors. Of course, these parts were almost always molded in color by the manufacturer, negating the need for a refinish process on them.

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But what about us, the repairers? I was told by the OEMs, "We don’t really have a process for that yet."

I remember the first time I received a door-trim panel for a Chevy pickup and it came in the wrong color — after I’d ordered it according to the interior trim code. I called the dealer to re-order and was told it was a "paint to match" item. My paint jobber was just as mystified as I was. We had the interior formulas but no process for painting those early parts. I don’t know if you’ve noticed, but a door-trim panel on a Chevy pickup can take a little bit of a beating from its drivers. But due to the lack of proper refinishing methods, many of those early painted plastic parts looked sad in a relatively short period of time. We tried everything our inventive minds could come up with, but we didn’t have the primers, additives and flexible agents that are now available in the aftermarket.

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I’m happy to say those days are gone. But even with the very capable assistance of the paint manufacturers, our troubles are far from over. We now have solutions for most of our plastic-painting problems, but the process isn’t foolproof by any means. It requires a good deal of diligence, investigation and close scrutiny of the paint manufacturer’s recommended procedures to be successful. I equate success with producing a repair that will be equal in longevity to a lifetime warranty. This is superior to what the OEMs offer at the time of vehicle purchases, but it’s common practice for many repairers in today’s marketplace.

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Plastic Troublemakers
Where do all these problems stem from? Let’s start with the different types of plastics. Why are there so many? Mainly because they’re used for different reasons and in different ways. For example, look at the traditional GM urethane bumper cover. It’s flexible, resists a certain level of impact very well and is usually assembled over a reinforcement-bar setup. It normally comes primed but requires the addition of a flexible additive to the paint system to maintain the finish flexibility the OEM intended. Ford, on the other hand, builds a face-bar setup that’s basically one component. The outer cover and reinforcement are integral and create one piece that’s rugged and has almost no flexibility. These are just two of the many bumper systems from two of the many OEMs — and both require two different methods for successful painting. And this requires technicians to use a variety of methods for a successful refinishing job.

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Making Sense of Plastics
I divide the many types of plastics into just two groups. No, I’m not a chemist and I can’t read down a list of plastics abbreviations and give you their technical names. I’m simply an automotive repair technician who follows a repair-procedure system developed through both successful and unsuccessful experiences, extensive training and strictly-adhered-to paint manufacturer’s recommendations. Perhaps you can minimize the unsuccessful experience part by obtaining good training (I-CAR plastics repair course, paint manufacturer training, etc.) and closely following recommended procedures.

That disclaimer stated, let me proceed. I divide raw plastics into two categories: those that require a polyolefine/polypropylene-type primer on bare plastic and those that require urethane and other types of adhesion primer. I don’t consider the abbreviations for plastic types to be at all useful. They’re not universally used and can be somewhat misleading. Just remember, if you have the need to paint bare plastic, it needs to be primed for adhesion purposes.

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Polyolefine/polypropylene has become more common because the excess that’s generated in the molding process is easily recycled with very little waste. Other types of plastics aren’t as easily recyclable. However, polyolefine/polypropylene has presented some adhesion problems in its bare plastic state. Some paint manufacturers have developed a specific primer for adhesion purposes for these plastics. Other paint manufacturers redesigned a singular adhesion primer to work on all raw plastics. The system we use has two different primers because our paint manufacturer feels that polyolefine/ polypropylene presents a special adhesion problem and deserves a primer of its own.

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I use a simple test to determine what primer to use. I cut off a sliver of plastic from the damaged part, clean the dirt and paint from it, and place it in a cup of water. If it floats, I use the polyolefine/polypropylene-type primer; if it doesn’t, I use the urethane-type adhesion primer developed for other raw plastics.

A burn test is another method of identification. When your sliver of clean plastic burns clean, it indicates you should use a polyolefine/polypropylene-type primer. When it burns with heavy black smoke, it indicates you should use urethane-type adhesion primer. These primers are for raw or bare plastic only and aren’t meant for use on painted or primed parts.

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Prepping Plastics
How you prepare the raw plastic for primer is also of the utmost importance. Wash the plastic with hot water and soap, then use a static-reducing wax and grease remover to remove any mold-release agents or other contaminants on the bare plastic.

Also, if you determine the part to be a polyolefine/polypropylene-based plastic, the sanding or abrading procedure is different from other plastics, which require a more aggressive sanding to achieve adhesion. Check your paint manufacturer’s recommended procedures closely.

Avoid prepping and sanding plastic parts more than eight hours prior to being painted. The surfaces of these plastic parts are designed to be flexible and, once sanded, they begin to "heal" themselves. The sanding "tooth" achieved by the process of abrading is significantly diminished after eight hours and will reduce adhesion capability, so wait until just prior to painting to sand or scuff the parts. This also goes for raw, primed or painted parts that are to be refinished. After sanding and abrading the raw plastic parts, they need to be degreased again with a static-reducing wax and grease remover.

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It’s important to clean and degrease all plastic parts. Some parts are primed at the factory using reversible primer (soluble in solvent) to improve the adhesion of topcoats. In most cases, these primers should be cleaned only with water-based wax and grease removers. If the part has a non-reversible primer, it should be cleaned with a static-reducing cleaner. Under no circumstances should you use traditional wax and grease removers on plastic parts! The rubbing and sanding process generates large amounts of static electricity in plastic parts, which attracts dust in the spraying process. In addition, these static-reducing traditional wax and grease removers can ignite in the presence of a static spark. Most static-reducing wax and grease removers are very aggressive, so it’s wise to pay attention to the reversibility of the primer on factory replacement parts to save yourself additional work. Also, pay special attention to the necessary evaporation time of these static-reducing cleaners. They’re slow drying.

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Be Flexible
Most primer surfacers that work on steel will work on properly prepared and primed flexible plastic parts. So, if you have a need to fill surface imperfections with a surfacer, do so with the addition of a flexible additive to your system’s surfacer. (Notice the word "system." I hope everyone’s using a complete system and not mixing one manufacturer’s surfacer with another’s flexible additive. It’s not a wise move — unless you like problems.) A flexible additive should be added according to the manufacturer’s recommendations.

The reason for adding flexible agents to paint is to keep the surface flexible according to the manufacturer’s intent. Some plastic parts are very rigid and require no flexible additive; some are very soft and require a lot of flexible additive. Every paint manufacturer has specific recommendations for the percentage of flexible additive to be added to primer, surfacer, sealer, basecoat, clearcoat and single-stage, depending on the degree of flexibility you’re trying to obtain. Follow instructions — the quality of the job depends on it.

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Quality flexible additives have elastomeric resins in them similar to what the OEMs use. There are cheaper substitutes on the market that allow a level of flexibility for the assembly of parts (face-bar covers), but they get increasingly brittle with age, don’t withstand an impact without fracturing and don’t duplicate factory-designed flexibility.

Step-by-Step Success
The products and processes used to refinish plastic parts have come a long way since I started in this business. But even with the advances made by paint manufacturers, our troubles are far from over. Properly identifying the type of plastic you’re dealing with and then adhering to the product manufacturer’s recommendations will help in your refinish endeavors.

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To prevent adhesion and cracking problems when refinishing plastic parts, remember to:

  1. Identify the plastic substrate using a float or burn test.
  2. Wash the part with hot, soapy water and then dry.
  3. Clean and degrease the part with the proper wax and grease remover.
  4. Sand and abrade the part.
  5. Clean and degrease the part again.
  6. Apply the proper primer and flex surfacers to raw or bare plastics.
  7. Apply sealers and topcoats with additives to achieve flexibility.

Adhering to the above procedures will help ensure success every time you refinish those once-troublesome plastic parts. And a quality refinish on any plastic part damaged in a collision will further enhance the overall quality of your repair.

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Contributing editor Mike West has been a shop owner for the past 25 years and is also a technician with 34 years experience. His shop in Seattle, Wash., has attained the I-CAR Gold Class distinction and the ASE Blue Seal of Excellence.

Photos taken by Eileen Benedict.

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