Introduction: Repairing Fiberglass

About: I used to work for, now I just make stuff. // follow me to see what I'm up to:
This Instructable comes from a recent project I have been working on that involves fixing up an old fiberglass travel trailer. In the remodeling of this trailer, I was left with many small holes from antiquated peripherals on the sides of the trailer (like a phone jack). There was also a six inch crack in the front from a rock hitting it on the road.

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

DISCLAIMER: Working with Fiberglass is rough. It is nasty nasty stuff, and can get into your skin, your lungs, and your eyes. When working with these materials it is important to wear a respirator, gloves, and safety glasses. It is advised that you wear a paint suit to cover all skin that could be exposed to fiberglass particulate.



Odds and Ends:

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Step 2: Surface Preparation

We went through and identified all of the holes that we needed to patch, and most of them were on vertical surfaces. With the permanent marker, we circled about an inch around the perimeter of each hole and crack. Then, using the angle grinder with a good grinding wheel, we ground down the surface to taper from thick to thin.

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

How many layers of fiberglass cloth you use depends on how thick your fiberglass shell is. Since I was trying to repair a 1/8" wall, I thought 3 layers of fiberglass material would be appropriate. 

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

When you glass in the patches, think of it as making a sandwich. The resin is your bread, and the fiberglass is your meat and cheese and fixins. You start with a paintbrush and apply a layer of resin to the area which will be receiving the patches. Then paint your largest patch with resin and smoosh it on to your prepared surface. Coat your next patch with resin, and apply it directly on top of your previous piece. So on and so forth until you have sufficiently stepped down to your smallest patch.

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 had drilled some small holes to sink some U-bolts into them last year, but wanting a clean start, I decided to patch these as well. Using the remaining catalyzed resin I didn't use for the patches - I mixed in some of the chopped fiberglass flake. After that was well mixed, we backed all the holes with duct tape and then began to fil with fiber flake, and sealed with a single small fiberglass sheet patch.

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.

That's it!  

I will continue posting Instructables about this project as it progresses, stay tuned!

Please read:
Removing Automotive Adhesive
Removing Rivets