3M defines large damage repair as any collision repair where the original paint layer has been broken and the damage includes significant denting. Successful major dent repair requires more than just filling, sanding and painting. It calls for a careful blend of skill and knowledge at each stage. This 3M Standard Operating Procedure (SOP) for large damage repair provides that knowledge, helping ensure that your success continues after the vehicle leaves your shop – with repairs that maintain OEM appearance and integrity over time. Our step-by-step guide on how to repair big dents on a car provides established best practices for preparing metal surfaces, from pre-cleaning to final sanding and inspection, before primer application.
Download English Large Damage Repair Standard Operating Procedure (PDF, 173 KB)
One of the most important times for cleaning the vehicles that need to be repaired is prior to initial disassembly. Pre-washing can be done efficiently in any shop, and can make subsequent cleaning steps faster and easier. 3M recommends pre-washing with a waterborne surface cleaner. Follow that with a silicone-free grease remover to eliminate any additional contaminants. Include all repair areas, jambs and adjacent panels. 3M also recommends power washing the vehicle undercarriage at the areas of repair.
Sand the repair area using a dual action (DA) sander with a grade 80 abrasive disc. DA sand to remove all paint at the area of repair, and extend the sanded area 2 to 4 inches beyond the damage. It’s important to provide this extra surface space for body filler and glazes, as applying filler or glaze beyond the base coat edge can result in repair mapping and costly comebacks. Blow off the entire vehicle with clean, dry compressed air. Include recesses, door jambs, wheel well areas and anywhere else that dust and particulates can settle. Re-clean the entire area with surface cleaner.
Small amounts of paint or other coatings can remain on the panel following initial prep sanding, especially in “low spots”. To remove them it is important to choose abrasives that are aggressive, yet leave a smooth finish. 3M recommends removing all remaining contaminants using a Scotch-Brite™ Clean and Strip abrasive disc. Next, use a 3-in. grinding disc to remove weld nuggets and other surface imperfections. Blow off the entire vehicle with clean, dry compressed air and re-clean it with surface cleaner.
Mix and apply filler per the manufacturer’s recommendation, or use the 3M™ Dynamic Mixing System (DMS). The DMS provides precise mixing of the body filler and hardener, and can virtually eliminate pinholes in the body filler. DMS also lets you apply filler directly to the filler spreader without a mixing board. Once the filler is mixed, apply a tight coat first to ensure the filler is fully wetted-out. Then build up the area with thin wet-on-wet coats. Make sure to keep the body filler within the primer feather-edge area to prevent repair mapping. Cure the body filler 15 to 20 minutes at 75°F/24°C.
Block shape sand the body filler using an 80 grade abrasive. After the initial sand, apply 3M™ Dry Guide Coat to highlight remaining imperfections such as scratches and pinholes in the repair area. Block sand the filler again. If necessary, re-apply the dry guide coat and continue hand block sanding to remove defects as required. Next, rough feather-edge the repair area using a DA sander and an 80 grade abrasive.
Final block sand the body filler using a 150 grade abrasive. Apply 3M™ Dry Guide Coat to highlight remaining imperfections in the repair area. Continue block sanding with the 150 grade abrasive and reapply the dry guide coat if necessary. Next, fine feather-edge the repair area using a DA sander and a 180 grade abrasive. If necessary, blow off the repair area with clean, dry compressed air, re-apply the dry guide coat and continue DA sanding to remove any defects as required. This step will leave a refined finish for primer or glaze (if glaze is required).
Blow off the repair area with clean, dry compressed air, completely removing sanding dust from the surface. Mix and apply polyester glaze per manufacturer’s recommendation, or use the 3M™ Dynamic Mixing System. Once the glaze is mixed, apply a tight coat first to ensure the glaze is fully wetted-out. Then build up the area with thin wet-on-wet coats. Make sure to keep the glaze within the primer feather-edge area to prevent repair mapping. Cure the glaze for 15 to 20 minutes at 75°F/24°C.
Apply 3M™ Dry Guide Coat to highlight imperfections. Block sand the polyester glaze with a 180 grade abrasive. Re-apply glaze and/or dry guide coat and block sand as necessary. This step will leave the damaged area level and ready for primer application.
Blow off the entire repair area with clean, dry compressed air. Feather-edge the area surrounding the filler and glaze using a 180 grade abrasive. Be sure to remove any remaining straight-line scratches from the repair area and abrade the outer perimeter for primer. Inspect the entire repair for quality.