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PAINTING PLASTICS: A Recipe for Success

Paint failures associated with plastic parts outnumber those on steel panels. Is it product failure or an application error?
Thursday, October 1, 1998 - 00:00

Because plastic (or composite) parts constitute an increasing percentage of today's outer body panels, a painter has to be prepared to apply coatings to these panels efficiently and without the risk of failure. Unfortunately, paint failures on plastic parts account for some the most common problems we see in the application of automotive coatings in the aftermarket. These failures are preventable, however. Plastic bumper covers can be restored to their pre-accident flexibility, appearance and durability.

It'll All Come Out in the Wash

Training in the application of paint to these parts is essential. Kurt Basil, collision marketing manager for Sherwin-Williams, says that successful painting of these parts often boils down to one essential--making sure the part is clean. The release agents used in the injection molding of these parts is the most common culprit in paint adhesion failures. Petroleum-based release agents, soap types and the internal silicone forms of these chemicals all must be completely removed prior to painting. Toyota, for example, changed its mold lubrication from a wax type to soap type agent, and use of a solvent-based cleaner prior to a simple soap-and-water cleaning of a bumper cover could possibly drive the stuff down into the pores of the material. In such an instance, the paint application could fail regardless of a thorough soap-and-water wash.

Always use soap and water first with a gray scuff pad. Some painters like to use a cleanser like Comet for this job. Basil reminds us, "Does the water bead on the part? If it does, you're not finished cleaning--we are not always spending enough time with the soap and water. And be sure to clean the back side of the part as well, to prevent transfer of the agent from the back [of the cover] when handling the part."

How'd That Happen?

When one looks at the typical paint failure on a plastic part, there are patterns that become evident (see illustrations). For one, the adhesion problems arise in areas near indentations and creases. Clearly, the individual responsible for paint prep didn't thoroughly scuff the part for the paint work. There was poor adhesion in areas where the scuff pad couldn't reach. And judging from the state of the bumper covers on this Pontiac, no form of adhesion promoter was used to give the color a bite into the plastic substrate.

In another view of the rear bumper cover it appears the bumper may have experienced a minor bump. (These things do happen--after all, they are bumpers.) Now some may claim that their paint work wasn't meant to be hit or abused, and no one can expect a body shop paint job on a thermoplastic olefine (TPO) bumper cover to hold up the way an OEM coating would. However, if we are truly restoring these cars to pre-loss condition, this also means we have to restore the previous level of durability.

To learn more about how to prevent these failures, Sherwin-Williams' research and development people worked on golf carts constructed entirely of TPO. Like most paint manufacturers, SW has developed a mid-coat adhesion promoter that has been tested and proven (according to SW) to provide adhesion on these parts equivalent to that of the OEM coatings.

Rob Gray, an expert with Sherwin-Williams who has done a significant amount of research into the problems associated with refinishing plastic parts, offered some insight into the real-world processes. "The factory process cannot be duplicated in the body shop," Gray said. "Temperature is the primary difference between the OE methods and those used in the body shop. [However], we can duplicate the factory finish in the performance testing with the application of 1 to 1.2 mils of adhesion promoter, 1 mil of basecoat, and 2 mils of clearcoat."

"We supply to a lot of companies that do work for OEs directly, and TPO is low-cost plastic, so more and more are using this. Because of this, we needed a proven product for this application. Our testing of the adhesion promoter involved scoring of the paint film to break it open. Then the part was subjected to water immersion, after which it received a thermal shock where it was frozen to -20 degrees. Then we hit it [at a 30-degree angle] with a pressure washer at 3,000 psi. The paint on the cover survived perfectly--time after time. We also did long-term weather testing with Florida exposure."

Sherwin-Williams developed both a clear and a black version (Plastic Adhesion Promotor UP07226 and UP07227) of a universal adhesion promoter for use with their paint systems. Gray tells us these products will work on virtually any plastic part found on a car, including the TPO bumpers. (And it's a safe estimate that aftermarket bumper covers suffer three times the adhesion problems of their OE counterparts.)


When is a bumper not really a bumper? Can you flex these parts after they've been painted? Can you bump a bumper, and have it live to tell the story? The vast majority of bumper cover paint failures involve some alleged minor impact. In my travels I see a lot of older body repairs, many of which look as good as they did when they left the shop. But the refinish work on the urethane bumper covers invariably fails before the rest of the car. Why is this? Odds are the employees who painted these parts didn't prepare them properly, mix the proper flex additive, or use any sort of adhesion promoter prior to paint application.

Anyone who has had to work with some older fleet vehicles knows the problems associated with multiple coatings on flexible parts. Gray remarked, "As you increase film thickness, you decrease flexibility. It's a real problem." Gray acknowledged the problems associated with attempts to re-coat bumper covers that have already been painted in a body shop. Salvage parts, for example, should be rejected if they have ever been repainted because the time required to strip these parts is not cost effective.

The Heck With Flex?

Given the constraints of the competitive body shop business, in which techs are always under the gun to beat the clock, conditions are not always conducive to doing everything by the book. For example, in order to properly apply paint to flexible parts, the painter must mix the appropriate flex agent with the color and/or clear. However, this means mixing more than one batch of material for the typical job. Ideally, the bumper covers are painted separately while off the car. But this isn't always convenient or, in fact, possible.

If, for example, the rear half of the quarter panel was damaged and requires an application of basecoat color, you can bet the rear cover needs to be blended to ensure an adequate color match. But this means the cover has to be removed, or at least detached from the body and masked. Then a separate application of color and clear has to be made to the flexible part. You're no doubt asking yourself, "How often does this happen?"

Over the years we have found the urethane color systems to be extremely flexible and durable--so flexible, in fact, that many painters are omitting the flex additive indicated by the paint manufacturer. It's all about time: the flex additive slows the drying process and adds several steps to the paint process. So we're finding it just ain't happening on a consistent basis--and, as a consequence, we're seeing far more plastic paint failures down the road.

The good news is that there are clears--Sherwin-Williams' CC639, for example--that work on flexible parts without the need for a flex additive. As a vote of confidence in these clears (when used properly), SW even offers a refinish warranty on the materials.

The bottom line: Plastic parts can be restored to their former glory. It just takes a little time, attention to detail ... and a bit of good old elbow grease.

Editor's note: The author would like to thank Sherwin-Williams for its contributions to this story. This article does not in any way endorse the use of a particular company's products.


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