How To Repair Plastic Auto Parts

Fixing damaged plastic bumpers involves grinding, sanding, sculpting and painting. But it's worth the effort for repairs that would cost less than your deductible.

There oughta be a law: If there's a post in a parking lot—any post—it should be high enough to see when you're backing up. Unfortunately, the law we usually see applied is Murphy's, and the resulting body-damage repairs will cost just a few dollars less than your insurance deductible. And that's precisely the scenario we were faced with after trying to maneuver our trusty Honda Civic out of a tight space in a crowded mall parking lot.

A lot of people would first try to do what you shouldn't: Head to the local auto parts store for some el cheapo body filler and a can of matching spray paint. Surprise! The technology of auto body parts has changed—and you'll need a completely different array of supplies to repair painted plastic parts than you would use to repair sheetmetal. Fortunately, there are a number of companies making repair and refinishing materials for plastic bumpers, as well as plastic fenders and door skins made from sheet molding compound.

Scuff, clean and prep area with plastic prep solvent to remove dirt and oil, and promote adhesion.

Bumper Mechanics

Today, the actual "bumper" part of a front or rear bumper is invisible. The part of the car's structure that's used for saving the vehicle's body from damage is well hidden behind a fairly large semirigid piece of painted plastic. This plastic covering is designed to deflect and deform easily when bumped. The theory goes that you can run into it lightly and the plastic will pop back into its original shape. However, really wallop it, especially with a hard or sharp object, and the plastic will abrade, cut or tear.

Various carmakers use different methods of attaching these plastic bumper covers. Because they are held in place with screws and tabs that fit into slots in the body sheetmetal, you may have to do some hunting around for all the fastening points of a typical fascia.

On our damaged Civic, for example, there were screws behind metal caps near the trunk latch, as well as screws under the taillights and at the wheel wells. Once they were removed, we were able to slide the bumper's tabs off a number of slots in the rear quarter panels. Look for screws or bolts under the trunk carpeting between the wheel and the rear bumper. You may also find fasteners hidden beneath or behind the bumper fascia.

Grind a "V" into the back of panel to remove loose material and give more surface area for adhesion.

Fascia Facelift

You'll need a warm, dry place to work because the repair adhesives and fillers cure within a fairly narrow range of temperatures. If you have the fascia off the car, you can work indoors—but not in the basement because these chemicals can give off some pretty noxious fumes. Read the labels carefully.

Not all plastic bumpers are made from the same stuff. But with the fascia removed, it's easy to identify the type of plastic because it's stamped into the back side. Stamps of PP (polypropylene), PPO (polyphenylene oxide) and TPE (thermoplastic elastomer) indicate plastics that smear easily when ground or machine sanded. (Our Civic bumper is made of polypropylene.) Stamps of PUR (polyurethane plastic rigid) and TPUR (thermoplastic polyurethane elastomer) are plastics that powder when ground or sanded.

The type of filler you use to repair the bumper depends on the plastic type, and there are many different manufacturers. Consult with the counterman at the auto paint store for the appropriate products. While the specific brand you use isn't critical, it is important that you stay with that same brand throughout the entire repair to be sure the products are compatible.

Regardless of plastic type, prepping the fascia is the same: Begin by slightly scuffing the damaged area and cleaning it with plastic surface cleaner. If the bumper is cut or torn through completely, make sure to scrub the inner and outer surfaces. After rinsing and letting the surface dry, wipe the area with prep solvent. When wiping with this material, go in only one direction because going in two directions simply drags contaminants back into the repair.

Thoroughly mix equal amounts of hardener and repair adhesive on a piece of cardboard or paper.

After the solvent has dried, sand the area by hand with 80-grit paper. Next, you'll need to form a "V" groove in the damaged area on the front and back sides of the fascia. The grooves allow you to align the two edges more easily. They also provide more surface area for the repair material to adhere to.

For plastic types that powder when sanded, cut the grooves with a 24-grit disc on a sander. For plastics that smear when sanded, make the grooves using a cordless drill and a rotary file. This gives you greater control and slower speeds to reduce the chance of heat-generated smears.

Spread plastic filler over the repair. Don't slop it on heavily or you'll have more work later.

Repair tears and cuts from the back side of the fascia. If the cut or tear is large, you might have to align and then hold the edges of the repair with masking tape on the front of the fascia. Use self-stick fiberglass-repair tape to add structure to the cut or tear. Next, mix the adhesive and apply it over the tape with a body-filler squeegee. When the first layer of tape is in place, apply a second layer so the threads run at a 90-degree angle to the first layer.

Once the material has hardened (in about 20 minutes), move to the front of the fascia and remove any tape (if applied). Next, using 80-grit sandpaper, remove any material that has squeezed through, and sand any spots that are above the finished level of the fascia. Fill the cut or tear and any low spots in the front of the fascia with the appropriate repair material, and squeegee it level. After it has hardened, sand everything level, first with 80-grit, then 120-grit, and then apply a light skim coat of repair material to fill pits and surface imperfections, and to restore the original contour. Then finish sand with wet 400-grit paper.

Sand and contour plastic body filler with 80- and 120-grit, then finish with 400-grit waterproof sandpaper on a rubber block. It will probably take two coats to restore contour properly.

PAINTING THE PLASTIC

When the repair is completed, apply two wet coats of flexible part sealer. After drying for 30 minutes, the fascia is ready for priming and painting. Again, read the labels and follow the maker's recommendations for operator safety—this stuff can be very toxic if inhaled. Modern primers and basecoat/clearcoat paints are flexible enough to accommodate plastic bumpers without peeling or loss of adhesion. Prime the fender with two coats of any two-part primer-surfacer, making sure to let the primer dry between coats. Once the primer has hardened, dry sand the repaired areas with 400-grit paper to level it and remove any imperfections.

Before spraying the bumper with basecoat, wetsand it and gently wipe the area with a tack rag to remove dust, then spray according to the manufacturer's instructions. You may need to repeat this process two or three times to cover completely. Once the basecoat is dry (usually about 30 minutes), mix the clearcoat with hardener. Apply two medium clearcoats, allowing each to dry in between.

After drying overnight, the fascia is ready to be reinstalled. Once it's in place, lightly wetsand the painted surface with 1500-grit paper to remove dust particles and surface paint imperfections. Avoid sanding through the paint by staying away from creases and seams. When done, dry the fender and look for shiny areas, which indicate that more sanding is needed. To bring out the luster of the paint, use rubbing compound on the foam polishing pad of a rotary buffing machine, still avoiding creases and seams. Get the final sheen using a fresh foam pad and polishing compound.

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