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Painting plastic parts

Special procedures and precautions set painting plastic apart from conventional techniques
Monday, May 13, 2013 - 08:54
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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. 

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