Training ground: Getting a grasp on bumper repairs

For the past seven years, manufacturers have improved plastic adhesive repair products to the point that they have virtually eliminated both air and airless plastic welding, processes that never lent themselves to production bumper repair. The once cumbersome and confusing process of identifying the type of plastic a part is made from has been greatly simplified. Finding the ISO identification mark, which sometimes is very difficult, is not as critical as before. Even the difficult — and in one case dangerous (burning) — tests that were once recommended have been simplified.

That means that many of you who once avoided this type of repair, should rethink your philosophy. If you're looking for some instructions, refer to the following steps.

STEP 1: Decision, decisions. Depending on which plastic repair system you select, your most difficult choice may be whether to use an adhesive promoter. I reviewed a number of systems and asked the obvious question, "If I use an adhesion promoter on plastic that doesn't need it, will it jeopardize the repair?" I discovered that none of the system manufacturers say it will. Considering the importance of cycle time, this green light reduces the complexity of plastic repair.

As you settle on a particular system, make sure you use it exactly as directed by the manufacturer. Never mix and match components from different systems. Something as simple as using one brand of adhesion promoter with a different repair system may cause the repair to fail.

STEP 2: Keeping it clean. Plastic parts typically have normal-use grime on them and may too, depending on its age, have mold release residue that must be cleaned off. Thoroughly clean the plastic with automotive soap and water, plastic wax and grease remover. Perform a rinse test. If a plastic part has been cleaned properly, the rinse water will "sheet" off instead of beading.

Make sure the tools and supplies for the job are gathered together (see Fig. 1). This will speed up the repair.

STEP 3. Reinforcement. The backside of the repair must be reinforced. This can be done with cloth mesh or wire material, as well as a plastic adhesive reinforcing patch (red). (See Fig. 2).

Over the years different types of reinforcement materials have been used, from nylon to stainless steel. Today most repairs are completed with cloth or the plastic patch backing. If you are concerned with strength, however, using either the steel screen or the plastic patch is best.

To repair a bumper that has been damaged through the complete thickness of the plastic, all of the damaged areas should be removed, allowing the plastic to lie in its original shape. To help the plastic "remember" its original shape, use a heat gun to warm the plastic, (about 150 degrees F). When the plastic is straight and shaped correctly, the part can be prepared for patching.

(See Fig. 3. Note that all the damaged plastic has been removed, allowing it to lie in its original shape.)

STEP 4: Bevel and clean. Remove the paint and bevel the edge, so the filler plastic will have enough thickness to bond correctly (see Figs. 4 and 5).

Using an abrasive pad, remove all the debris from the beveled area and clean with a plastic cleaning agent.

When cleaning plastic, keep in mind that if standard wax and grease removers are used to clean the raw plastic, the plastic may swell, making it nearly impossible to complete a repair that isn't visible. Note too that plastic also is very susceptible to static charging. Cleaners designed for plastic generally have anti-static agents.

The back should be beveled and cleaned as well. Apply an adhesive promoter before applying the patch to assure good adhesion of the plastic bonding compound. Most manufacturers provide adhesion promoters in aerosol cans, which will speed up the process considerably.

STEP 5. Patch things up. Plastic patches are fast and very strong. All you need to do is remove the backing and apply the patch to the prepared area (see Fig. 6).

If the patch is not used, you must add reinforcement to the backside repair. Place a temporary patch such as aluminum tape on the outside so the adhesive bond does not fall through (see Fig. 7). Apply the adhesive. Apply the reinforcement into the soft adhesive bond.

Make sure that the bonding plastic comes through the mesh of the reinforcement (see Fig. 8). When the backside plastic has cured, remove the temporary patch from the front. Apply the plastic bond and flatten it with a spreader.

STEP 6: Looking good. The plastic filler, when cured, can be blocked to level, primed, and finished. To be sure that the patch will be strong, make sure that the mesh reinforcement is covered well and that it extends far enough over the area being repaired to provide the added strength. The plastic adhesive should fully cover the reinforcement without dry areas or places where the mesh is not fully embedded in the bonding. The front should be "fit up" so some of the bond comes through, similar to a weld.

Plastic bonding should be spread over the beveled outside, completely covering the beveled area and extending past so it can be feather sanded to a level surface when cured (see Fig. 9). If necessary, apply a second coating of bonding filler to help obtain a smooth and invisible repair (see Fig. 10).

Conclusion

Today, plastic repairs use the same kinds of skills we have been using to apply plastic body filler for years. By following just a few simple steps, you can complete a two-sided plastic bumper repair in short order, with confidence that it will restore the bumper to its pre-accident condition.