The days of only occasionally painting plastic in the body repair shop (Fig 1) have passed. Nearly every job now requires some sort of plastic refinishing. Though painting plastic uses similar processes to refinishing other substrates, some special considerations and precautions must be taken to insure a high-quality repair job.
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 are different for refinishing undamaged, previously finished products.
The stages of painting plastic, while similar to painting steel, include 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. Here is a rundown of those steps for success.
Identify the plastic
Before a painter can develop a paint plan for a plastic part, he 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 is a plastic that, once it has hardened or cured, will not soften when heated.
Many flexible parts on a vehicle, such as front and rear fascia, are made of a thermoplastic material; when heated, this material will become even more flexible. In contrast, mirrors and grilles are made with a thermoset plastic that does not soften when heated. 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).
Plastic parts have a plastic identification ISO code molded into the back that identifies the type of plastic (Fig 2) the part was made from. When using parts made from olefin polymers, the technician must use an adhesion promoter before refinishing to assure that the finish will not delaminate later. Once the type of plastic has been determined, the paint plan can be made. 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.
Make a repair plan
The painter's repair plan will differ depending on the type of plastic part being painted, first by identifying if there is the need for adhesion promoter, such as with polyolefin parts, which need an adhesion promoter applied to all exposed surfaces. Therefore a new plastic part will be prepared differently than a new primed plastic part, or repaired plastic part, or than one previously finished. New primed plastic parts, where all the plastic has been completely covered with a primer from the manufacturer, does not need to have an adhesion promoter applied, even if it is manufactured from polyolefin. However, if the polyolefin was exposed, an adhesion promoter must be applied prior to priming.
Steps for primed plastic parts: inspection, washing (soap and water wash) and cleaning (wax and grease remover), scuffing, sealer, basecoat, clearcoat and detail for delivery.
Washing with soap and water is important when painting any type of substrate, but plastic parts seem to be especially subject to contamination, and the washing should be done with great care. The two-bucket method of washing, with one bucket of warm soapy water for cleaning and a second with clean, cool water for rinsing, helps to eliminate any debris in the washing bonnet so as not to scratch the vehicle in the process (Fig 3). The soap used in the soapy bucket should be PH-balanced automotive soap, and the water should be as warm as possible. If the cool rinse water becomes overly dirty, it should be discarded and replaced with clean water.
Cleaning involves the use of a chemical cleaner such as an approved wax and grease remover. This should be applied and wiped off, not allowing the wax and grease remover to dry on the surface. If the wax and grease remover is allowed to dry prior to removing it, the contaminants will return to the surface.
Scuffing of the primed plastic part can now be done, with a gray or gold Scotch scuffing pad (red is too aggressive). The use of scuffing paste can be very helpful when preparing plastic parts; the paste is applied to either a wet or dry scuffing pad (Fig 4), and the entire area is scrubbed to abrade the surface. The sanding paste makes a foamy cleaning substance that evenly abrades the surface, preparing it for paint. Following scuffing, the part must be thoroughly rinsed (Fig 5). If not rinsed properly, some sanding pastes can leave a milky residue behind on the plastic part. The part should then either be allowed to air-dry, or should be mounted on a holding device for painting and dried with compressed air.
Sealing. Once the plastic part is placed in the paint booth, cleaned (Fig 6), and tacked (Fig 7), it can be sealed. When sealing a plastic part, its orientation is not relevant, and because gravity can help get the sealer into the uneven surfaces on many plastic parts, it can be oriented upright (as seen in Fig. 8). In this position the paint can more readily be delivered into the grill area, lights and other uneven surfaces.
Basecoat can be applied once the sealer has flashed for the paint manufacturer's recommended amount of time. A sealed part is first tacked, and then the part is oriented to the same position it will be in when mounted on the car (Fig 9). Though it is not as easy to get the paint into the uneven surfaces in this position, it will help considerably with metallic orientation. The metallic lie is more likely to match correctly when painted in position.
Clearcoat. Following the recommended flash time and proper tacking, the part can be reoriented upright before the clearcoat is applied. By applying clearcoat in the upright position, it again helps with good gloss and flow-out in the hard-to-reach areas (Fig 10).
Detailing, after the proper curing and or baking, the part can be detailed for delivery. If good cleaning practices have been followed, little or no buffing should be necessary.
Steps for New Unprimed Parts: inspection, tempering, washing (soap and water wash) and cleaning (wax and grease remover), adhesion promoter, scuffing, sealer, basecoat, clearcoat and detail for delivery.
Inspection. All new parts for the day – plastic, aluminum, or steel – should be inspected upon arrival at the shop. Check for damage, scratching, and other conditions that might require repairs. Also check before painting to make sure that the part is the correct part and will fit the intended vehicle. Then remove the protective coating (Fig 11), and with a gloved hand (Fig 12), closely inspect for imperfections. If imperfections are found and they cannot be sanded out, they should be repaired before proceeding to the next step.
Tempering. Some paint makers recommend that new raw parts be baked in the paint booth at 140F 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.
Washing and cleaning. Though the process of washing and cleaning a new, unprimed part is similar to the process mentioned above, new unprimed plastic parts often are contaminated with mold release agents. Moreover, if the part has been tempered, which brings to the surface residual mold release agents, it's important to wash both the inside and outside of the new part (Fig 13). If the underside is not thoroughly clean, it is likely that through normal handling of the part during the painting process contaminants will be brought to the surface and can cause fish eye or other problems (Fig 14). One way to check that all the release agents have been removed is to drag your finger across the newly cleaned part and observe whether a trail of oil is left behind. If so, re-clean with both soap and water and wax and grease remover.
Scuffing. The scuffing process for new unprimed parts is the same as explained above for primed parts.
Adhesion promoter will need to be applied to all polyolefin plastic parts, as well as to many other highly flexible plastic parts. Adhesion promoters come in many forms; some are applied by spray guns and are often supplied as a ready to spray (RTS) product, while others come as aerosol applications. Still other paint manufacturers supply both a cleaning sponge and aerosol adhesion promoter for their products. Whichever you choose to use, you should follow the paint manufacturer's recommendation. Following the proper flash time, the part should then be primed and sealed according to the paint manufacturer's recommendation.
Basecoat, clearcoat and detailing steps are all identical to above.
Steps for Repaired Plastic Parts: inspection, washing/cleaning, adhesion promoter, priming/blocking, scuffing, sealing, basecoat, clearcoat and detailing.
Inspection. The painter must be sure that the surface of the repaired area is straight, level and without defects. Commonly the repaired area must be feathered, primed, and blocked, but before this can be done the part must be cleaned of contaminants.
Washing/Cleaning. Though the part may have been washed and cleaned prior to its repairs, it's likely that it has picked up contaminants during the repair process. Therefore, the same procedure of washing, then cleaning with a wax and grease remover should be followed before paint preparation of the gun.
Priming/Blocking. The repaired area is first sanded with a dual action sander (Fig 15); then the correct primer for plastic parts, with flex additive, if required, is applied. After the primer has cured, a guide coat (Fig 16) is applied, and it is box sanded (Fig 17) until all areas are flat, straight, and level – to assure an undetectable repair. The part is again washed clean with soap and water and cleaned with a wax and grease remover, then mounted on a stand and prepared for scuffing.
Scuffing, sealing, basecoat, clearcoat and detailing are the same as above.
Be prepared for static electrical charge
Plastic, both thermoset and thermoplastic, will quickly become statically charged. It is charged, for example, by rubbing cleaning cloths (both cloth and paper types) over its surface. It also will be charged by tacking. 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 avoid adding static charge, which will attract dirt. However, sometimes in spite of the painter's 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 add only 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. Another alternative would be to equip the booth with a grounding strap. A simple 12-gauge wire attached to the steel portion of the paint booth with a long enough lead wire to reach the steel bumper rack will supply a good ground. Paint technicians must use their experience and consider their options when deciding to tack after applying anti-static agents.
Painting plastic correctly the first time will help eliminate costly repairs later and will help earn customers for life.
