Thanks for the question. This sounds like a professional level repair so it may be best to take it to a shop that is proficient with plastic repairs. If the you decide to take it on yourself, follow these steps. With polyurethane you can eliminate the adhesion promoter steps.
Extreme Strength Plastic Repair Process
- Clean the Damaged Area. Clean the front and back of the repair area with soap and water, followed by a VOC compliant surface cleaner.
- Apply aluminum autobody repair tape to the front side of the repair to temporarily align and secure the damage while the back side reinforcement is being completed.
- On the back side, use a Dual-Action (DA) sander with grade 80 abrasive disc to thoroughly sand the repair area where the reinforcing adhesive will be applied. Tapering is not necessary on the back side. Blow off with clean, dry air and apply adhesion promoter, allowing 5 minutes to dry.
- Apply a 3-5mm thick layer of 3M 4247 or 4248 Super-Fast Repair Adhesive to the back side extending 50mm beyond the damage in all directions. (Work quickly, the adhesive will begin to gel in 20 seconds at 75F.) Allow set-time of 5 minutes at 75F (24C) before continuing to the next step.
- Remove the aluminum tape. On the front side, use a grade 50 3″ disc or grade 36 or 60+ file belt at a low speed to create a gradual taper 3″ wide. Grind the taper deep enough to expose a ¼ in. wide strip of the previously applied Super-Fast Repair Adhesive on the back side. The 1/4″ strip of adhesive should be visible through the center of the damaged area.
- Use a DA with grade 80 abrasive disc to create a smooth transition into the tapered area. Remove any melted plastic and create a fuzzy surface for the adhesive. No shiny plastic areas should remain. Blow off the front side repair area with clean dry air, apply aerosol adhesion promoter and allow to dry for 5 minutes..
- Apply #4247 or #4248 Super-Fast Repair Adhesive into the taper with an initial “tight coat” immediately followed by additional adhesive to fill and level or slightly lower than the surrounding area. Allow 15 minutes to cure at 75F.
- Use a DA with grade 80 abrasive disc to sand the Super-Fast Repair Adhesive slightly lower than the surrounding surface to allow for finishing with a cosmetic adhesive. Use a DA with grade 180 abrasive to further refine course scratches where the adhesive will be feathered.
- Blow off the front side repair area with clean dry air. Apply aerosol adhesion promoter and allow to dry for 5 minutes. Apply # 5887 EZ Sand Multi-Purpose Repair Material with an initial “tight coat” immediately followed by another coat to level the surface. If step 8 is done carefully, only a skim coat will be required. Allow 15 minutes to cure at 75F.
- Use a DA sander to finish sand the repair area using grade 180 abrasive disc followed by grade 320 abrasive disc. Blow off and inspect the repair for quality. Any small defects or pinholes should be filled with # 5887 EZ Sand Multi-Purpose Repair Material. Polyester fillers can cause bubbling and should not be used.