Repairing Fiberglass





Introduction: Repairing Fiberglass

About: I'm an Instructables success story! After relying on the site to DIY my way through art school, I was able to join the Instructables Design Studio in 2012. It's the best! Whe...

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.

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!



  • Small Disposable Paintbrush
  • Angle Grinder with Grinding Disc
  • Orbital Sander with 80 Grit Disc

Odds and Ends:

  • Stir Sticks
  • Respirator
  • Gloves
  • Safety Glasses
  • Permanent Marker
  • Duct Tape
  • Rags

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



    • Oil Contest

      Oil Contest
    • Water Contest

      Water Contest
    • Creative Misuse Contest

      Creative Misuse Contest

    21 Discussions

    once the fiberglass is layed up, it should immediately be rolled out with a fin or bubble roller to remove any air trapped in between layers or part. air bubbles reduce bond and will expand with heat, causing future problems. these air bubbles (air bombs) reduce strength of repair and can also fail leaving a hole in glass and paint or gelcoat

    the smaller patch should be applied first, next size up secondly, then largest third. this gives you more bonding layers. this isnt so much an issue with a minor repair, but when doing larger repairs using technical fibers, you would want your layers staggered. this is so each layer has bonding area, and the ones on top bond to the layers underneath as well as the outer circumference.

    Hey, trailer is from my hometown... what are the chances.


    1 year ago

    Sorry to say, I got the horse before the cart. I did some fiberglass work on my motor home. Today I put some reisin and mesh on, checked it three hours later some was tacie and some was as wet as when I put it on. I did what I should have done before beginning, looked at your web site. My dilemma is, what do I do now?


    1 reply

    Ugh! That can be a real mess! Give it a full 48 hours to potentially set. It sounds like your catalyst wasn't completely mixed it, or there wasn't enough, or maybe it went bad?

    If it still isn't set, you can clean up uncured resin with mineral spirits (with the right PPE on, duh!).

    I'm guessing you'll have to grind out what you patched and begin again :-/

    Fiberglass is pretty forgiving, just a mess to work with.

    Wd40 works as a very effective releasing agent, if you don't want the resin to stick to something. Talcum powder mixed with the resin and catalyst is a very effective filler for small holes.

    If the surface is vertical, and your patch large so its weight would allow it slip down, I use two sided tape to stick the DRY cloth onto the hole, then use a paint brush to paint the resin onto the cloth.

    And talking about hulls, anyone have thoughts on applying gelcoat so it is a perfect smooth surface?

    1 reply

    Hi there,as to applying gelcoat smoothly. Depending on the size of your repair,first give the area a light sanding then you can mix however much gelcoat you need,what I do is use the two inch celotape packing tape. I put the gelcoat over the spot then apply the tape,it smooths out the gelcoat and puts it in right where you want it.Let it cure 24 Hours just to be sure then take the tape off. There might be a little more sanding but not near as much as if you SMUUUDGED it all over! lol

    If you want a properly strong and flexible finish, try this: since glass fibre mat is made up individual fibres thinner than human hair which are bonded together to create strands, the correct method is to stipple the mat with resin & a brush until the strands are broken down into the fibres and each fibre is consequently surrounded by resin. The strength of GRP comes from the fibres not the resin, and so long as the fibres are surrounded by resin you will achieve the lay-up. Consolidating the area with a metal roller will make the whole thing far stronger and also bring excess resin to the surface. (Don't forget to clean equipment in acetone for next time!)

    When wetting your patches, do it on a sheet of polyethylene (US spelling) sheet of some sort, such as the corrugated plastic campaign signs that are everywhere here. Don't do it in your hand. I would prepare a series of circular patches but I'd place the smallest first, and after painting the area to be patched with the resin. Smallest-first gives you flat, parallel layers.

    Also, the warnings about curing may be overdone. Once catalyzed, resin is gonna cure. Slow or quick, but it'll cure. Letting sun hit it is also a catalyst. The fumes inhibit curing, and are heavier than air so if you are working in a bowl shaped place, you need a breeze to move those gases out.

    Hi, when laminating a hole and where you have access to the outside of the repair, you should use a temporary backing to which the glassfibre will not bond. Eg a bit of polyprop plastic covered with polythene or cling film or a specialist non-stick material called peel ply. If doing this, you can lay some gel coat (to give a good finish to the outside of the repair), before adding the fibreglass layers. Hope that makes sense.

    Am a fiberglass moulder (lamination) I want a job in London

    I'll have to give this a shot at work. We' have some minor cracks and dents in the glass that could use some fixing up. If the problems keep up, we'll have to get a professional eventually. Otherwise, this doesn't sound like that bad of an idea to get fixed. I'll pass it to my superior and hopefully he'll approve this project.

    Thanks for the clear instructions, I repaired a large pot in a fountain after reading this.

    Went to my Blog, and see there, also, 2 Posts down, about Sandables: