Painting plastic parts

Jan. 1, 2020
Many of the techniques that are used for painting plastic parts are extremely similar to the way other things are painted every day.
Figure 1

What is so different about painting plastic parts (Fig 1)? Don't you use the same procedures and techniques as you do when you paint other parts of the car? To those who may wonder, the answer to that is both yes and no. Many of the techniques that are used for painting plastic parts are extremely similar to the way other things are painted every day. The parts need to be cleaned, sanded and scuffed, primed or sealed, inspected for any imperfections, have basecoat applied then be clear-coated. So what's the big difference?

Although painting plastic employs many of the same techniques used when painting other types of materials, there are also special procedures and precautions to use when painting plastic to insure a long lasting quality finish. In fact, almost every automotive or paint

Figure 2

manufacturer provides special instructions that should be followed when finishing plastic. These special instructions differ for finishing new unprimed plastic, new primed parts, repaired plastic and also for refinishing undamaged, previously finished products. The stages of painting plastic, while similar to painting steel, involve many specific 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.

The type of plastic being painted will vary, and identifying which type of plastic you'll be painting is critical for a good paint job. 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

Figure 3


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 that does not soften when heated. Parts can also 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 identifies the type of plastic the part was made from. Parts that are made from Olefin Polymers must have an adhesion promoter used before refinishing to assure that

Figure 4

the finish will not delaminate later.

Painting new primed plastic parts
Many of the steps that will be taken in painting parts are similar for all the different types of plastic substrates. Cleaning is one of them, and it is one of the more critical steps when dealing with plastic parts. During the manufacturing of all plastic parts, a substance known as mold release agent is applied so that the new plastic part can be easily removed from the mold it is cast in. And though this mold release agent is very helpful when manufacturing a new part, it can be very troublesome when it comes time to paint that part.

As with any surface preparation, the first step is soap and water washing (Fig 2). 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 part, it should be washed both inside and out, to avoid transferring the contaminants later when moving the part. The soap should be a Ph.-neutral automotive soap to avoid contaminants found in other types of soaps. To be sure that the part has had all the mold release agent removed, the technician should take note of the clear water that he or she used for rinsing the part. If the water beads (Fig 3), all the release agent has not been removed, and the part should be rewashed. But if the water sheets off (Fig 4), it's likely that all the release agent has been removed, and the technician can proceed to the next step. That step is chemical cleaning; plastic parts should be cleaned with an isopropyl alcohol, which will remove any non-water-soluble mold release agents. The third step 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, when the part is dry the technician should use a gloved finger to lightly drag the surface for about 6 inches. If contaminants remain, a trail from the finger will be noticeable, and the cleaning process should be repeated. 

Figure 5

Plastic substrate is one of the softest materials that technicians repair for paint, and special care must be taken when choosing the abrasive for this process. 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 also 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 grit. A better method, though, is to use a gray abrasive pad with sanding paste. (Red abrasive pads are too aggressive for soft

Figure 6

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 lubricates 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, (Fig 5) the part will not take on a static charge during the scuffing process.

One of the more critical steps with plastic preparation, especially after using a sanding paste, is the removal of all residue from the sanding process. The part should be rinsed thoroughly with large amounts of warm water, using a clean cloth to go over the scuffed areas, making sure that no sanding paste film or sanding residue remains. The part should also be dried,

Figure 7

which is often done with compressed air — although my choice has become an air amplifier (Fig 6), the type that is used for waterborne paint; it seems to dry the parts quicker. The prepared part should have a clean, uniformly dull sheen. If any shiny spots remain, the scuffing should be repeated. When sanding has been completed, the part should be placed in a holding fixture (Fig 7) so that 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. Parts sprayed out of position may reveal metallic orientation problems once they are attached on the vehicle. Often when the metallic orientation is different, the paint that would otherwise match well may not appear to

Figure 8

match properly. A parts stand that holds the part in its proper position will help with correct metallic orientation when the part is placed on the vehicle.  Not all holding fixtures are created equal, and although they are designed to hold most bumper configurations, it is always tragic if a part, for whatever reason, slips out of the holding device. Therefore it has become my practice to use plastic tie wraps to secure the part in place, just to be sure (Fig 8). 

This new, primed part is now ready for sealing. The part should be thoroughly inspected for any imperfections that may have been missed. Then, if there are no spots that have broken through, the sealer should be applied according to its manufacturer’s recommendations. Flex

Figure 9

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 added. 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. 

When applying any coating to a plastic part (Fig 9) that has many curves, indentations or even holes for lights, it can be difficult to apply the coating evenly. 

FIgure 10

Application of color coat and clearcoat
After the sealer has been allowed to flash its recommended time, a color coat can be applied. Though the application process of applying color coat to a plastic part is no different, it is sometimes more difficult. The part has many varied surfaces that can be difficult to paint with a uniformly even coat. When applying any coating to a plastic part (Fig 9) that has many curves, indentations, or even holes for lights, it can be difficult to apply the coating evenly. In addition, good gun control is critical for uniform color appearance and proper metallic orientation (Fig 10). Making sure that the gun is held perpendicular to the part being painted requires dexterity, as the different levels of the surface result in many position changes. 

Figure 11

One of the techniques used by many techs to assure that the deep cavities have sufficient coating is to apply the coating, at least partially, from the back. This should also be done when applying color coat (Fig 11).

Once the base coat has flashed, application of clearcoat can be performed. As has been mentioned, getting a uniform covering to such a diversely shaped object can be difficult. Because at this point metallic orientation is no longer a concern, the part can be sprayed in an upright position, which allows gravity to help coat the inner surface better. Most good holding devices are capable of letting the part be moved into a horizontal position, and the clearcoat is

Figure 12

then applied with somewhat less difficulty as gravity helps it flow (Fig 12).

Painting new unprimed parts
Although the painting of a new, unprimed part is very similar to that of painting any unprimed part, the main concern becomes its cleaning and scuffing. An unprimed part (Fig 13) also is more likely to have mold release agent remaining from its manufacturing. It is cleaned using the three-step method mentioned above, but special care must be taken to assure that the cleaning is done completely, both inside and out, checking carefully for any residue that might be left. Once the technician is a sure that all mold release agent has been removed, he or she can proceed to the scuffing process. A technique sometimes used to bring any mold release

Figure 13

agent to the surface for easier cleaning is to bake the new part in the paint booth at 140° for 40 minutes. This helps consolidate the mold release agent, and subsequently it is easier to clean.

Scuffing is done in the same manner as explained earlier, with gray Scotch-brite and sanding paste, and is rinsed and cleaned with large amounts of water, leaving the now slightly rough finish that an adhesion promoter can be applied to.  Although not all plastics require an adhesion promoter, most bumpers are made from polyolefin plastics, which do. Identification of the plastic is required to determine whether an adhesion promoter is necessary. 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 painting.  Depending on the paint system that you're using, you should read and follow the instructions provided by the paint manufacturer for the proper application of adhesion promotion. 

If no repairs are necessary, then sealing, basecoat application and clear application are done as outlined earlier.

Figure 14

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 (Fig 14). 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 (Fig 15) 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

Figure 15

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 applied. The part can then be sealed as needed, and then color- and clear- coated in the normal fashion.

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. They are the baking of new plastic parts and tacking the part after the spraying of anti-static agents. Two cautions should be included in this area: the first is that when cleaning raw plastic, which includes newly repaired raw plastic, a wax and grease remover specifically formulated for cleaning raw plastic should be used. Wax and grease removers, which are used normally on non-plastic substrates, are easily absorbed by raw unpainted plastic. This causes the plastic to swell, which will result in contra mapping (bull's eye) following refinish.

The second caution regards static electricity. Plastic, both thermoset and thermoplastic, will quickly become statically charged. It is charged by rubbing cleaning cloths (either cloth or paper) over its surface. It will also be charged by rubbing a tack cloth over it. Some painters will spray a plastic part with anti-static agent after it has been completely cleaned and tacked, and then never tack it again throughout the painting process. By not tacking it, they do not add 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 to tack after applying anti-static agents.

Though it might seem at first that painting plastic correctly is a long and complicated process, once a standard operation procedure is set up and the shop has all the necessary products at hand, the process will actually go very quickly. Painting plastic correctly the first time will eliminate costly repairs later and will help promote customers for life.

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