Plastic fenders on ATVs, motorcycles and automobiles can suffer any number of damages, particularly impact damage from abuse or collisions. Depending upon the material make-up of the plastic, it can be repaired in a number of ways by using different products and applications. The trick is to repair the crack or split with a product and technique that offers the best reinforcement and strength. Kits are available that ensure proper bonding and holding strengths that can keep the crack from reopening. The average vehicle owner can follow a few simple techniques and use some basic tools to repair his own damaged bumper.
Remove the bumper from the vehicle with the appropriate socket or end wrench. Set a plastic tarp on the ground and place the bumper on it. Use a drill motor and a very small bit (1/8-inch) to drill holes around the perimeter of the crack or split. Space the holes about 1/8-inch apart, surrounding the crack on all sides. Widen the crack opening by using the drill to cut a channel. Do this by drilling a hole in the end of the crack and slowly drag the bit down its length. Wipe away the residue with some alcohol.
Use heavy 60-grit sandpaper to sand the inside and outside of the bumper. Sand one inch past the outside of the patch area. Use a crosshatch sanding pattern, sanding at 90-degree angles. Rough up the plastic surface with deep grooves. Wipe the bumper down with the plastic cleaner (from kit) or use alcohol. Let the bumper air dry.
Cut a piece of fibreglass mat that will cover the crack and overlap the drill holes by at least 1/2 inch. Prepare the adhesive according to directions and pour enough of the contents into a cup to fill the entire crack, with an extra amount to use for build-up. Soak the fibreglass mat in the adhesive mixture and lay it flat over the crack on the underside of the bumper. Use some blue masking tape to secure it tightly against the bumper, but do not cover the fibreglass mat entirely with tape–only the outside edges. Let dry.
Turn the bumper over and fill in the crack gap with repair plastic. Smooth it out with a putty knife, shoving down hard to remove all air bubbles. Build up an extra layer of repair plastic beyond the height of the bumper surface. Let dry according to instructions.
Flip the bumper over, backside up. Remove the blue tape. Cover the fibreglass mat with a thick coat of repair plastic, smoothing it down with the putty knife. Leave a thick build-up area of extra plastic putty. Let air dry.
Turn the bumper face up. Use a flat file or sanding block with 60-grit sandpaper to sand the surface down until you reach just above the bumper height. Use fine sandpaper (400-grit) to finish sanding the surface down until it becomes even with the bumper surface and contour. Paint to match your bumper's colour, as needed.
Work in a well-ventilated area and wear gloves or a particle mask if you are sensitive.