Editor's note: This article was originally published Aug. 1, 2006. Some of the information may no longer be relevant, so please use it at your discretion.
Although painting plastic incorporates many of the same techniques used when painting other types of materials, there are special procedures and precautions used when painting plastic to insure a long lasting quality finish. In fact, almost every automotive or paint manufacturer provides special instructions that should be followed when finishing plastic. These special instructions differ for finishing new unprimed plastic and repaired plastic, and also for refinishing undamaged, previously finished products. The stages of painting plastic, while similar to painting steel, involve many special steps and products that must be used to insure the type of high quality and longevity that is demanded in today's collision repair market.
Before a painter can develop a paint plan for a plastic part, he or she must identify what type of plastic was used to make that part. Though we use the general term "plastic" to identify many different non-metal parts, there is a staggering array of plastics used in manufacturing a vehicle. The two main categories of plastics used are Thermoplastic, a type of plastic that will soften when heated, and Thermoset, which after it hardens or cures, will not soften when heated. Many flexible parts on a vehicle such as front and rear fascia are made of a Thermoplastic material that when heated will become even more flexible. In contrast, mirrors and grilles are made with a Thermoset plastic
Parts also can be made from many different compounds such as Thermoplastic Olefin (TPO), Polyurethane (TPUR), Acrylonitrilebutadiene-Styrene (ABS), Sheet Moldable Compound (SMC) or Fiber Reinforced Plastic (FRP), just to name a few. Plastic parts have a plastic identification ISO code molded into the back that identify the type of plastic from which the part was made. Parts that are made from Olefin Polymers must have an adhesion promoter used before refinishing to assure that the finish will not delaminate later.
Once the type of plastic has been determined, the tech can decide the proper painting approach for the item. Painters may be called upon to paint plastic parts that are in differing conditions such as new unprimed parts and new but primed parts. They also will need to know that refinishing a repaired plastic part and finishing an undamaged part each require different preparation steps and precautions
New Unprimed Parts
Painting new unprimed plastic first requires a thorough cleaning of the part: soap and water washing, removing mold release agents, chemical cleaning and inspection are all parts of the pre-sanding cleaning. The part must be sanded and cleaned, then anti-static and/or adhesion promoter is applied. The part is then sealed, color coated and clearcoated.
After inspecting a new part for damage, it must be cleaned to remove any contaminants deposited during manufacturing. Plastic parts, unlike steel parts, often are contaminated with mold release agents. Mold release agents are lubricants that are injected with the plastic media during manufacturing to help extract the part from the molding dies. Because these agents are mixed with the plastic pellets during manufacturing, they cover the entire surface of the part, inside and out. To remove these agents, the initial cleaning is an important step prior to painting.
Cleaning
Cleaning new, raw plastic parts is a three-step process. As with any surface preparation, the first step is soap and water wash. The difference with plastics parts is that the water should be hot; one paint manufacturer recommends that the water be as hot as the technician can stand, to help dissolve the water soluble contaminants. In addition, because mold release agents are on all surfaces of the parts, it should be washed both inside and out to avoid transferring contaminants later when moving the part. The soap should be a pH-neutral automotive soap to avoid contaminants found in other types of soaps.
The next step is to clean the part with isopropyl alcohol, which will remove any non-water-soluble mold release agents. Once the technician has completed step two, the final task is to clean with a wax and grease remover to remove other non-mold release agent contaminants.
Remember that a thorough cleaning of both the inside and outside of the new part is necessary to prevent re-contaminating the surface of the part.
After the three-step cleaning process, the part should be inspected for cleanliness. If you suspect that the part is not completely clean, the steps should be repeated.
There are two indicators that help a technician know whether or not a part is completely clean:
- When rinsing the part following cleaning, if the rinse water beads instead of flows over the part, then contaminants remain.
- After the part has been cleaned with all three steps and has been dried, the technician should place a gloved finger on the part and lightly drag it for about six inches. If contaminants remain, a trail from the finger will be noticeable, and the cleaning process should be repeated.
Sanding
After the part has been thoroughly cleaned it should be scuffed. A common mistake when preparing plastic parts for paint, especially soft Olefin plastic parts, is the choice of abrasive. While thorough sanding is essential, the choice of abrasive is critical. Avoid using coarse and aggressive paper: P-320 or even P-400 is far too aggressive for soft plastic surfaces. If you choose to use paper, it should be P-800 to P-1000. A better method, though, is to use a gray abrasive pad with sanding paste. (Red abrasive pads are too aggressive for soft plastic.)
There are many sanding pastes to choose from, but the types that are designed specifically for plastic scuffing are the best. The sanding paste helps lubricate the gray pad as it is used, lengthens the pad's usefulness and helps keep the surface clean as the part is scuffed. By scuffing the surface wet with sanding paste, the part will not take on a static charge during the scuffing process.
After the part has been completely scuffed, it should be rinsed and dried. (Compressed air will speed up the drying process.) The dry surface of the part should have a clean and uniformly dull sheen. If shiny spots remain, the scuffing should be repeated.
When sanding has been completed, at this point the part should be placed in a holding fixture so it will be painted in the same position as it would be on the vehicle. This procedure is used to assure that any metallic in the paint will be sprayed in the proper orientation, thus producing a better color match.
Application of Anti-Static and Adhesion Promoter
After a part has been scuffed and placed on a painting fixture, it should be cleaned again with wax and grease remover. This part should then be allowed to completely flash after cleaning and should have a uniformly dull and chalky appearance.
Because plastic easily takes on a static charge and plastic parts with a static charge attract dust, an anti-static agent should be sprayed on. The next step is to apply adhesion promoter to the prepared plastic. Any plastic made from Thermoplastic Polyolefin is inherently difficult to paint. Because adhesion is difficult, all Olefin parts must be sprayed with an adhesion promoter, which will allow the plastic and a coating applied over it to coalesce, or fuse together. Olefins that are not treated with an adhesion promoter often result in the finish delaminating in large sheets following refinishing.
Priming or sealing
The plastic part is now ready for priming or sealing. As with other surfaces, the choice between a sealer or a primer surfacer is made on the basis of defect. If the new part is free of all defects, the part can be sealed, then topcoated. However, if the part has defects, the parts should be blocked to remove the defects before topcoating.
Flex additives are agents that can be added to a coating to increase its flexibility, though there has been some controversy regarding their use. Some argue that when used, flex additives only provide extra flexibility to the topcoat for a short time, and that if the part is painted on the vehicle, flex additives are not necessary. Others state that the flexibility remains for long periods of time, and, therefore, all coatings should have flex additives. I have even heard that if flex additives are added to primer or sealer, they make the coating a chip-resistant coating (However, I have not been able to find a single paint manufacturer that supports this claim).
The safest recommendation regarding flex agents is that all paint manufacturers' recommendations should be followed. If your system recommends that flex additives be placed in the primer, sealer or basecoat before application to a flexible part, that guideline should be followed.
Application of Color Coat and Clearcoat
At this point, a plastic part has been cleaned and scuffed, has had anti-static and adhesion promoter applied, and has had the manufacturer's recommendation for flex additive followed. The application of color coat and clearcoats proceeds the same as when applying it at any other surface. A sprayout panel should be made to verify a blendable match; the technician should apply basecoat to full hiding; proper flash times and cleanliness measures should be heeded. Then the clearcoat is applied in the normal fashion.
Preparation of New Primed Plastic Parts
Though new primed parts do not need the application of an adhesion promoter, they do need to be cleaned using the three-step method (hot soapy water, isopropyl alcohol, and wax-and-grease remover). The primer should be scuffed with a gray abrasive pad and sanding paste, water-rinsed, and then re-cleaned with wax-and-grease remover. At this point the anti-static agent should be applied. If the part has no bare plastic showing through the primer, adhesion promoter does not need to be applied. The application of color coat and clearcoat then proceeds the same as on other parts.
Preparation of Repaired Plastic Parts
A repaired plastic part presents a challenge in that it is truly two different surfaces, the raw or newly-repaired plastic and the non-repaired previously painted part. The newly-repaired area on the part should be treated like a new raw plastic part. It should be triple cleaned, sanded and have the repair edge feathered as needed for application of primer filler.
If your paint manufacturer recommends the adding of flex additive to primer filler or if special plastic primer filler is recommended, the recommendations should be followed. Before applying the primer, adhesion promoter should be applied to the newly repaired area. It should be extended slightly over the feathered edge to assure full coverage of the raw plastic. When the repair has been blocked and the remaining areas scuffed and prepared for paint, the part should be cleaned and anti-static agent should be applied. The part can then be sealed as needed and color- and clearcoated in the normal fashion.
Preparation of previously finished parts
Previously finished parts do not need adhesion promoter applied if no defects are found when cleaning and scuffing, but these parts should still be cleaned thoroughly because the technician does not know how well they were cleaned before their original painting. They should still be triple cleaned, scuffed, re-cleaned, have anti-static applied, and then be painted normally.
Two controversial topics remain that must be covered when talking about the painting of plastic parts — the baking of new plastic parts, and tacking the part after the spraying of anti-static agents.
Baking
Some paint makers recommend that new raw parts be baked in the paint booth at 1,400F for 30 minutes before triple cleaning. It is believed that mold release agents that are close to the surface will be driven out so they can be cleaned away. One should remember, though, that Thermoplastic parts are the only ones that baking will affect. Thermoset plastics, by definition, are not affected by heat, and baking would not help these types of plastic. Baking of every Thermoplastic part is not necessary, but if a lead painter notes that certain brand plastic parts are hard to sufficiently clean, in those cases baking may speed up the process.
Tacking after application of anti-static agents
Both Thermoset and Thermoplastic plastics will quickly become statically charged, perhaps by rubbing cleaning cloths (both cloth and paper types) over the surface. Plastic also can be charged by rubbing a tack cloth over it. Some painters will spray a plastic part with an anti-static agent after it has been completely cleaned and tacked, then never tack it again during the painting process.
By not tacking it, they avoid adding a static charge, which will attract dirt. However, sometimes in spite of efforts to avoid it, a plastic surface will collect particles that require tacking during the operation. If tacking is necessary, very light tacking between color coats may only add a minimal amount of static.
As an alternative, a steel parts stand that is grounded on the steel grates of a booth floor may help eliminate any charge that may be added by tacking after the anti-static agent is applied. In booths with no steel grate, the technician could wet the concrete floor so the stand would be grounded. Paint technicians must use their experience and consider their options when deciding whether to tack after applying anti-static agents.
Though it might seem that painting plastic correctly the first time is a long and complicated process, once a standard operation procedure is set up and your shop has all the necessary products at hand, the process will actually go very quickly. Painting plastic correctly the first time will help eliminate costly repairs later and promote customers for life.
