Spraying composites can be a big challenge unless you take the right approach.
So what types of problems do painters face with painting plastics that they don't see with other substrates? Surprisingly, many of the problems that come with plastics are the same or similar to the problems painters have when painting steel and other substrates. With plastics, the painter must be especially thorough. Other difficulties are specific to the plastics themselves, and require either a different process or different product.
This article will examine plastic identification, choosing proper cleaning agents, cleaning, prepping different types of plastics, priming and sealing, along with part orientation. We'll also look at painting various plastic parts.
Plastic identification
Identifying the type of plastic that new non-primed parts are made from may be important when choosing the type of adhesion promoter a painter will use. Some manufacturers specify one type of adhesion promoter for parts made from the differing types of Polyolefin, often marked as PO, PP, PE, PP/EPDM, PP/EPM or TPO. Manufacturers will recommend a different type of adhesion promoter for other types of plastics, marked SMC, BMC, PC, ABS and TPU (to name just a few). Polyolefin plastic parts often are flexible ones that need to be flexed when installed or be able to rebound from minor damage while in use. The other groups of plastic parts are generally rigid parts, though this criterion alone does not identify the plastic.Plastic parts are marked on the back side with identifying abbreviations for the type of plastic that they were manufactured from. If the part cannot be identified, there are two tests that can be performed to identify different categories of plastics – the burn test and the float test. Few manufacturers recommend the burn test since it produces toxic fumes. The float test is accurate and safe.
To perform the float test:
- Shave off a sliver of the plastic part from an area that will not be visible. Make sure it is clean from paint, mold release agent and other contaminants.
- Place the sliver in a container of tap water.
- If the sliver floats, it is a polyolefin.
- If it sinks or submerges, it is not a polyolefin.
Why are polyolefins so significant? Generally, adhesion promoters are used on plastic parts manufactured from polyolefins. Some paint systems call for adhesion promoters for plastics made from non-polyolefins. Painters should know their paint system and follow the system manufacturer's recommendations to prevent failures.
Cleaning
The three guidelines for making a business successful sometimes are noted as location, location and location. When painting plastic parts, the first three guidelines are clean, clean and clean.
A part that comes to a painter arrives as either a new non-painted part (raw plastic), a primed part or a painted part. New, raw plastic parts are covered with mold release agent used during manufacturing to allow them to be removed from the mold after injection. The release agent is mixed with plastic beads. When injected and heated during manufacturing, the agent infuses the entire part. This process aids manufacturing, but for refinishers it can cause adhesion problems if the release agent is not completely removed.To force the infused agent to the surface, some paint systems recommend baking the part before final cleaning. The heat causes the release agent to migrate to the surface, where it can be cleaned off. This "tempering," once recommended by most paint systems, is not universally recommended currently. A painter should check the individual paint system's procedure sheet before heating a part. If heating is used, the part should not be draped over a part stand. Instead, it should be supported properly so it will not droop during heating, which can prevent the part from fitting properly.
Plastic parts should first be washed with warm water and automotive soap, both inside and out, then rinsed thoroughly. If the water beads up while the part is being rinsed, the technician should wash it again, then re-rinse the part. This washing and rinsing should continue until the rinse water sheets smoothly off the part. (One paint company recommends that a part be washed two to three times to assure that all the mold release agent is removed.)
Following the soap and water washing, the part should be chemically cleaned. Use caution when cleaning new primed plastic parts, as the primer must first be solvent-tested. Some new parts that come primed will feature a primer that will be washed off with wax and grease remover or isopropyl alcohol. On an area that will not show, the part must be tested with standard solvent wax and grease remover first. If the primer does not come off, then the product can be used to finish cleaning the part. If it does, a waterborne plastic cleaner should be used instead.
If the part is solvent-safe, it should be cleaned with a recommended plastic part wax and grease remover. Often the plastic wax and grease remover is a waterborne product. This removes any non-water-soluble contaminants that were missed with the first washing.Next, the part should be cleaned with an anti-static agent to help control the static electricity that quickly builds up on plastic parts. Often anti-static solutions are isopropyl alcohol products. To speed the process up, some manufacturers have one solution that is both a plastic-safe wax and grease remover and an anti-static agent.
New raw plastic parts, which are the most likely to be contaminated with mold release agent, should also be cleaned thoroughly. Parts that are repaired with raw plastic exposed should be cleaned with great care. Parts that come primed (both inside and out) and previously painted (with both sides covered with a coating) are less likely to have mold release contamination but should still be cleaned with care. Remember, the sheeting of the rinse water is a good indicator of how clean a part is.
Plastic surface preparation
With the part cleaned, it is time to prepare the surface. Raw or plastic parts or repaired parts with raw plastic exposed are prepared certain ways. These parts can be coated with either a primer or some other type of finish coat, which requires a different type of preparation.
New raw plastic or repaired plastic: With raw plastic exposed, the part should be tested to see if it is a polyolefin or not. If it is, the raw plastic must be treated with an adhesion promoter. Plastics parts that are not polyolefin may or may not, depending on the system that is being used, need an adhesion promoter. On repaired parts with raw plastic exposed, the adhesion promoter should be applied to the raw plastic area only (while the adhesion promoter may not harm the other areas it is sprayed on, this is a waste of the product).
The adhesion promoter process causes some confusion with painters. Some system manufacturers recommend using adhesion promoter for all raw plastics (thus eliminating plastic identification and testing), while others recommend identification or testing and two different types of adhesion promoters. The safest option a painter can take is to read, understand and follow the recommendations for the system in use.Flex agents also can be confusing because some systems recommend that all coatings used on plastics use flex agents, while others do not. Not all systems require flex agents in the base coat, and some don't even recommend using flex agent in the clear coat. Again, the best policy for a painter is following what is recommended by the system being used.
Care also should be taken when sanding or scuffing plastic parts. Soft flexible plastic parts can be easily damaged during the sanding process. Most coatings manufacturers do not recommend using aggressive sanding on plastic parts. In fact, either gray or gold abrasive pads are commonly recommended. If red pads were to be used, scratches could be left in the plastic surface and would be visible after painting.
A common recommendation for preparing plastic parts is using sanding paste. Such agents have both an abrasive and cleaning agent specifically designed for plastic surfaces. Some even come with special application brushes for hard-to-reach areas. It is important that all areas to be painted on the part be cleaned and scuffed well. Some plastic parts, because of their design, are particularly difficult to thoroughly prepare, requiring the technician to take special care during preparation.
Priming and sealing
When it comes to priming and sealing, once again, the procedure that a painter follows depends largely on the system being used. Some systems have a special plastic primer surfacer that may require a flex agent. Others may use the same primer surfacer with a flex agent, while others do not need a flex agent added. Parts that do not need primer surfacer should be sealed with the recommended sealer. This could be the standard sealer with or without a flex agent, depending on the system.
New primed parts often are cleaned thoroughly and scuffed, then sealed and painted. Though there are fewer steps with this process, it does not mean that they should be done with any less attention to completeness. Painting in general, and plastic painting in particular, demands that each step be done with care and completeness.
Part orientation
The painting of most plastic parts is performed with the part off the vehicle. With today's complex colors containing pigments, metallics, and mica, the orientation of the metallic and mica becomes critical. Parts to be painted should be mounted on a paint jig in the same orientation as they are when the part is mounted on the vehicle. With the parts in this position, proper metallic orientation can be achieved, so it will be less likely to have a color mismatch when the vehicle is reassembled for delivery.
Most paint jigs are moveable, which means that the part can be placed in the upright position following the application of basecoat. Some painters find they cannot produce a good clear coat application on complex plastic parts without either having dry spots or runs when sprayed in their proper orientation. By raising the part to the upright position, gravity can be used to the painter's advantage.
Some plastic parts are particularly difficult to polish. The painter should take every effort to avoid finish polishing or de-nibbing.
Curing
As mentioned earlier, plastic can move when heated. Therefore, if the part is to be baked for curing, care should be taken that it does not deform due to its not being properly supported during heating. Using a fast-curing clear may be a better choice than baking the part, thus avoiding warping when baking.
Conclusion
As designers change vehicles in an effort to make them more fuel efficient, more plastic or composites will be used in the manufacture of vehicles, and painters should be prepared.
