Thick and Nasty

Gel Coat Repair

How do I remove bubbles from my resin molds? Removing bubbles from your molds and casts are not difficult. By using a pressurized pot you can squeeze out all the bubbles until they are inperceptable even in the clearest glass type molds.

Gel Coat Repair

Regardless of the care taken in producing fiberglass reinforced parts, some of them will require repair. Defects are caused by operator error, defects in the mold, contamination, rough de-molding and impact and abrasion during assembly, handling, and storage.

When a defect is discovered, determine and eliminate the cause so that it is not repeated in the production of subsequent parts. Next, determine which of the three major repair types fit the defectmarred or slightly scratched area; deep defects which require routing; or damage to the laminate behind the gel coat.


1. Wet-sand the damaged area using a good quality 320 grit sandpaper followed by successively finer grits ending with 600. Use a sanding block when applicable and always sand in the same direction.

CAUTIONUse 320 grit only when the defective area is deep. Minor scratches can be rubbed out with 400 grit followed by 600 grit or rubbing compound.

2. Wash sanded area with clean acetone. Be sure it is completely free of sanding dust and grit both during and when the sanding operation is completed.

3. Use a good quality rubbing compound and buffer to polish sanded area. It is advisable to use a white or translucent compound especially with white or pastel colored finishes. Use liberal amounts of compound, minimal pressure and keep buffer moving to prevent heat buildup.

4. When the surface appears to have a dull gloss, use a polishing compound to bring the gloss close to the original gloss.

5. Wash compounded and polished area with clean acetone.

6. Apply an even coat of paste wax and hand buff.


1. Grind out the defective area with a sharp routing tool. Do not leave any undercut edges. Feather in the defective area.

2. Sand entire defective area with 280 grit dry sandpaper, feathering in the edges. Use an air hose to blow out the dust.

3. Wash area with clean acetone.

4. Catalyst and mix the gel coat to repair defective area. (Epoxy putty may be preferred especially if damaged are is in hull or area below water line.) A reinforced putty can be used to fill in areas where there is a deep defect (more than 30 mils) which does not affect the laminate strength. Follow with a patch. For reinforced putty, mix 10% filler-milled glass fibers, 1% thixotrope to resin or gel coat. Mix well until obtaining smooth, paste-like consistency. Then catalyze, fill, allow to cure and sand for spray patch.

CAUTIONUse the same production batch of gel coat applied to the part being repaired. Use 2 cc MEKP per 100 grams of gel coat. Hand stir for one minute prior to applying to repair area. Fill area and smooth with flat edge of putty knife. Leave slight rounded over-fill to allow for shrinkage of the patch on curved areas Cover with wax paper or cellophane to exclude air from contact with the surface. Do not work at surface temperatures below 60º F.

5. When patch is firm and free of exotherm heat, remove wax paper or cellophane. Permit patched area to cure for a minimum of one hour. Wet-sand with 320 grit sandpaper until rough surface of patch is removed. Proceed with 600 grit and wet-sand until smooth.

CAUTIONAlways use a sanding block and sand in the same direction. Continue to wet-sand until all shiny edges on patched area have been removed.

6. Wash area with clean acetone.

7. Inspect area for defects. If area is non-porous and free of blemishes, proceed with rubbing compound, liquid polish and wax as outlined in "Marred or Scratched Areas" Steps 3 through 6.

8. Depending upon the degree of surface irregularities, it may be necessary to repeat Steps 4 and 5 or reduce the viscosity of the gel coat consistency and apply with a touch-up gun. This is accomplished by reducing the gel coat with Max. 20% (wt.) styrene monomer. Catalyze mixture with 2% (wt.) MEKP. Apply the reduced mixture on the area to be repaired using a uniform, even spray stroke. Permit 5-10 minutes to set and then topcoat with a water soluble polyvinyl alcohol parting film. When the patch is cured, the PVA film can be removed with tap water. Proceed with Steps 3 through 6 as outlined in "Marred or Scratched Areas."