Plastic Repair Techniques

It’s a plastic world out there, and its automotive uses go far beyond just bumpers and fascias.

Plastic body parts can be found in many spots on today’s cars and trucks. When you first think of plastic parts, you probably think of the front and rear bumper covers, or fascias. And you would be right. As you know, even minor fender benders can easily damage these components. There are other components, too, that are made from plastics and the use of plastics is continuing to increase.

While sometimes it is more economical to replace plastic parts, on other occasions you’ll find the need to repair these pieces in your shop. Let’s take a look at some plastic repair techniques.

Type of Damage

Before you begin any plastic body repair, first determine the type of damage. Then, determine whether or not it is financially practical to repair it. Remember, composite materials can mask the severity of an accident. Adhesive bond lines, interior structure of the doors and the steel structures need to be inspected carefully to get a true damage assessment. Close inspection might require partial removal of interior trim or inner panels.

Once the type of damage has been determined, how much money, in time and material, will be required to do the repair? If it exceeds the cost of a new component, replacement should be your option unless extenuating circumstances exist (part not readily available, customer needs the car back quickly, etc).

The damage to a vehicle can take many forms. If one vehicle hits the rear fascia on another vehicle, that surface might just be scraped (a simple repair), or cracked and punctured, as shown in Figure 1 (a much more extensive repair). If such damage has occurred, a reinforcement panel, or patch, must be bonded on the back side of the component.

Repair Guidelines

There are many different types of plastics used on today’s passenger vehicles. These plastics are classified as rigid, semi-rigid and flexible. Regardless of the type of material damaged, the repair procedure is basically the same. The difference is the material used for the repair. The materials must be specific for each substrate—rigid repair material for rigid plastics, semi-rigid repair material for semi-rigid plastics and flexible repair material for flexible plastics. If you are not sure what type of material from which the damaged component is made, refer to the Chrysler Plastic Repair Guide (81-316-0711). This guide will also show you different manufacturers’ recommended repair procedures and materials to be used during the different types of damage outlined in this article.

The following are some helpful hints and suggestions for repairing plastic components:

1. When sanding, always be careful to not get the substrate too hot.
2. If the surface does melt, let it cool, then re-sand the surface.
3. Do not apply cleaners or solvents directly onto any sanded substrate.
4. If the surface appears fuzzy after finish sanding, block sand with 220 grit paper.
5. The repair material should be slightly higher than the damaged surface to allow for sanding and contouring.
6. When applying repair material in layers, be sure to fully saturate the reinforcing material.
7. To prevent further tearing, drill a 1/8 inch hole at each end of the tear.

Surface Preparation

The first step in any high quality repair is to wash the vehicle with hot, soapy water to remove all water soluble contaminants. These include dirt, brake dust, road salt and bird droppings.

The next step is solvent cleaning which will remove contaminants such as grease, oil, road tar, waxes and polishes. Wipe the surface with the solvent cleaner. Do not apply the cleaner directly on the substrate or sanded surface. Wipe the surface dry with a clean cloth (this is the only time a solvent should be used during the repair).

One-Sided Repair

If the surface is scraped, but not cracked or punctured, this is considered a one-sided repair. Perform this repair as follows:
Note: It is recommended that the part be left on the vehicle whenever possible. This will save time and the part will remain stationary during the repair, preventing misalignment and stress in the repair area.

1. After preparing the surface as outlined above, dish out the repair area with an 80 grit disc. The repair area should be beveled at a 20° angle, as shown in Figure 2. This action will increase the bonding surface (make sure there are no sharp edges as these edges might show through the repair).
2. Apply an adhesion promoter (depending on repair material manufacturer).
3. Apply a thin coat of repair material over the entire repair area.
4. Add additional repair material to build the repair area higher than the surrounding, undamaged area.
5. Allow the repair material to cure per repair manufacturers’ recommendations.
6. Rough sand the repair material with 80 grit sandpaper.
7. Finish sand the area with 180 grit sandpaper, followed by 220 grit sandpaper.
8. Wipe the area with a clean, dry cloth (do not use a solvent).
9. If there are imperfections, repeat steps 2 through 6.
10. Prime and paint the repair area per refinish manufacturers’ recommendations.

Two-Sided Repair

A two-sided repair is just that; both sides of the component must be repaired to fix the damage to the component.

Perform the repair on the backside first as follows:

1. After preparing the surface as outlined previously, scuff the backside with 80 grit sandpaper.
2. Cut a piece of fiberglass cloth to cover the repair area overlapping by 1".
3. Apply an adhesion promoter (depending on repair material manufacturer).
4. Lay the fiberglass on a clean surface; using a plastic spreader, apply repair material to both sides of the fiberglass cloth.
5. Apply the fiberglass cloth patch to the backside of the repair.
6. Using the spreader, apply additional repair material over the surface, completely covering the fiberglass cloth patch (see Figure 3).
7. Allow the repair material to cure (there is no need to finish the backside of the repair).

After the backside has been repaired, repair the front as follows:

1. After preparing the surface as outlined above, dish out the repair area with an 80 grit disc. The repair area should be beveled at a 20° angle, as shown in Figure 2.
2. Apply an adhesion promoter (depending on repair material manufacturer).
3. Apply a thin coat of repair material over the entire repair area.
4. Add additional repair adhesive to build the repair area higher than the surrounding, undamaged area.
5. Allow to cure per manufacturers’ recommendations.
6. Rough sand the repair material with 80 grit sandpaper.
7. Finish sand the area with 180 grit sandpaper, followed by 220 grit sandpaper.
8. Wipe the area with a clean, dry cloth (do not use a solvent).
9. If there are imperfections, repeat steps 2 through 7.
10. Prime and paint the repair area per refinish manufacturers’ recommendations.

In the above two-sided repair procedure, we detailed the use of the fiberglass to reinforce the backside of the repair area. If the repair area is substantial, the area can be repaired using a piece of similar substrate.

This procedure, which can be time consuming, involves cutting out the damaged area in a square or rectangular shape. The replacement piece is cut to the same size. Small squares of plastic material are attached to the back side with small screws. Then, the repair piece is then set in place with adhesive and screwed into place. After the adhesive has cured, you remove the screws and apply repair material over the damaged area. You would then follow the above procedures to complete the repair.