How to: patch holes in FG

Jim_in_Houston asked how to patch small holes in fiberglass on another thread and suggested I start a new thread, so here goes.

Tools required:
Electric or air grinder. I prefer the 4" or 4.5" grinders. Scissors, composite roller(s), paint sprayer (or paint brush will work in a pinch), orbital sander (or muscle power in a pinch).

I will assume the patch is made from the outside, but can be done from the inside if accessible.

Start by grinding the edges of the hole to a 12:1 taper. In other words, the fiberglass (FG) at the edge of the hole should be paper thin and taper to full thickness on a 12:1 slope. So, if the FG is 1/4" thick then the taper would be 3" wide all the way around the hole.

Next you need to back up the hole. Almost anything that is non-porous can be used. I use plexiglass, pieces of plastic trash cans, etc. Whatever you use, it should be able to conform to the contours of the surrounding area. Now thoroughly wax the backing (to prevent the resin from sticking to it). Either glue, tape, wedge or prop the backing in place.

Now, you need to cut the FG patch layers. Start with matt (epoxy compatible) that is just slightly larger than the actual hole in the composite. Then cut the next piece about 1" larger in diameter and so one until you end up with a piece the same size as the overall taper diameter (kind of walking the patch up the taper gradually). You should alternate between matt and fabric (woven roving), but should start and end with matt. The goal is to acheive a total thickness of the patch that about equals the original thickness. However, the final patch should be just slightly below the final gelcoat finish. You can use small pieces of FG to fill any low spots after the last piece is laid up.

Once you have cut all the FG pieces, lay them out in reverse order...smallest on top of stack. Now, using epoxy resin (I use #635 from US Composites, but the epoxy resin available at Home Depot will work) wet out the entire tapered area. Wet out the first (small piece and center it in the hole. Now completely wet it out. Do the same with all the pieces. You should use a roller to compact the laminate and expel trapped air. You should roll out the laminate every third layer max. Let the new FG composite patch cure.

Grind down high spots and fair out. I use epoxy with a sanding aid/thickener to fair out my patches, but Bondo can be used. Sand the patch smooth remembering to keep it just below the final finished gelcoat surface.

Finally, you must replace the gelcoat. I spray the gelcoat on, but it can be brushed (just takes more sanding). Tape out an area just larger than the overall patch. Sand the untouched gelcoat with 120 grit paper so the new gelcoat will stick (remove any wax BEFORE sanding). Custom match the gelcoat to your boats color. This is the most important and hardest part to a visually good repair. To get a true color, you should sand and polish out an area of your boat to bring the gelcoat back to the original unfaded color. You can buy pre-mixed gelcoat from CC, but it cost about $180.00/gallon with a one gallon minimum order. Mix a sufficient quantity of matched gelcoat to put on three or more heavy coats. Gelcoat is catalyzed, so the unused portion will keep (un-catalyzed of course). Once you are satisfied with the color proceed on.

If spraying, thin the gelcoat no more than 20% with Styrene (do not use Acetone as it breaks down the gelcoat and thus it will not shine up very well when complete). Catalyze the gelcoat (mix well) and spray over your patch. I use a circular spray pattern and apply the gelcoat in a circular motion...sort of like welding. The idea is to build up a heavy coat without causing heavy orange peel or runs. Give the styrene a chance to flash off between coats. Gelcoat is air inhibited, meaning the surface will remain tacky unless wax is mixed in prior to spraying. However, if you mix in wax, you must sand the gelcoat between coats if allowed to cure. If you do not use wax, the tacky surface can be removed with acetone prior to sanding. I usually catalyze gelcoat with about 15-20 drops of MEKP (depending on outside temperature) per ounce of gelcoat. Cure time can be varied with quantity of MEKP used. Make sure you build-up the new gelcoat higher than the final finished surface to allow the removal of material to achieve a smooth surface flush with original surface.

After the new gelcoat has cured, wipe off the tacky surface or wax with acetone. Spray a very light trace coat of black spray paint on the new gelcoat (tracer). This will help you see your sanding progress. Start sanding using 120 grit paper on a sanding block made of wood or some other rigid material. Do NOT use rubber sanding blocks. I initially use an orbital sander. Once all the tracing paint is sanded out, re-trace and switch to 220 grit paper and sand a larger area. Do this again with 320 grit paper sanding an even larger area. Then, go to 400 grit (you can stop tracing at this point). Then 600 grit. 600 grit is the minimum before polishing...you can go higher for even a better shine and finish. Polish by hand (hard) or with a buffer/polisher. Use course rubbing/polishing compound as gelcoat is much harder than paint.

Presto, the new gelcoat blends into the old with an almost imperceptible blend line. You will be amazed. I am sure I have left out many small details, so just ask.

I'd say that's a pretty thorough explanation, thanks David F!

I think it would be awesome to start a referencee section organized to access info like this.

Then great advice like this wouldn't be lost as threads disappear after time.

Just a thought.

"fairing" is a term to describe the process of bringing an uneven or rough shape back to its original shape or in conformance with the surrounding shape. In other words, it is what auto body technicians do when they use Bondo.

All the supplies are available (reasonably priced) from UScomposites