I actively searched for the best way to patch small holes in fiberglass on the web, and couldn't quite find concise answers - this lead to a more dedicated search. I ended up talking to surfers, sailors, and prop-masters about how they make repairs to structual fiberglass and came up with the following methodology.
Hope this is helpful, and I would love to hear back from the community on this.
Step 1: Materials and Tools
I got a bulk of my materials from Douglas and Sturgess in Richmond, CA - the folks at the store were incredibly helpful and super nice! THANK YOU!
- Fiberglass Chopped Strand/Random Weave Mat
- Fiberglass Flake
- Bonding Resin
- Resin Catalyst
- Magic Sculpt Polyester Puddy Resin/Hardener kit (for backfill of the gelcoat on the exterior of the hull)
- Mineral Spirits for cleaning excess
- Mixing Container (Be sure to get one with the volume measurements on the side of the container for easy mixing)
- Small Disposable Paintbrush
- Angle Grinder with Grinding Disc
- Orbital Sander with 80 Grit Disc
- Stir Sticks
- Safety Glasses
- Permanent Marker
- Duct Tape
Step 2: Surface Preparation
The edge of the hole should be a sharp edge, with a taper moving outward back to the normal shell thickness.
Step 3: Preparing the Patches
A sailor friend of mine informed me that I would need to glass in a large patch first, and sequentially step down to a patch the size of the hole. I used kid scissors that I didn't particularly care about to cut the patches.
The largest patch extended a little past the outlined perimeter, with the next two patches cut sequentially smaller.
Step 4: Mixing the Resin
I began working with resins regularly when working on my thesis in college. I learned some tips about resin curing from professors and other students. You must consider the environment that you conduct pours in. The temperature should be relatively stable around 70 degrees Fahrenheit, and have a good air flow. Ideally, you are inside under a vent hood. For these pours, I was outside in the cold Oakland air, on a kind of damp day......not ideal.
If it is cold, or if you are making thin layers of resin, you will need to use more catalyst than is recommended. Often resin manufacturers will list how much catalyst is needed per volume of resin on the container. In this case, I used 1/4 oz. catalyst to 1/2 qt. resin. And then many extra drops for good measure, as it was cold and I was only making a thin layer.
CAREFUL, if you over catalyze your resin, it can become brittle or crack. BUT, if you do not use enough, your resin could have extremely long cure times, or not ever cure completely. ALSO, when adding catalyst to resin, it is important to stir while you are adding it, and for 60 seconds after it has been completely added. Scrape the walls and the bottom while stirring.
Have I scared ya? I didn't mean to. It's easy, just follow the instructions on the resin container and use your best judgement. (Here is an excellent tip sheet too.)
Step 5: Glassing in the Patches
If you have unwanted catalyzed resin drips on the other side, those clean up quickly with Mineral Spirits on a rag. Just be sure to get to them before they harden.
Step 6: Filling Small Holes.
I would liken it to smearing a wound with antibiotic ointment, then putting a bandaid on it.
Step 7: Backfill, if necessary.
This is when the Magic Sculpt comes into play, we used it like you would use a wood filler on a hole in lumber. It is a 2-part clay that needs to be mixed thoroughly for a few minutes to work effectively. We made 2 long ropes, then twisted those together, then twisted that rope, and kept twisting until it was more like mixing and squishing.
The putty was used to fill a few un-even pits in the gelcoat of the fiberglass from the rough patching we had done.
Once the putty had set up, we were able to sand it down to a smooth surface.
I will continue posting Instructables about this project as it progresses, stay tuned!
Removing Automotive Adhesive