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Fixing Fiberglass Flaws??

My car's fiberglass body is showing blisters on the surface. When sanded down it looks like someone applied filler over the original surface and that caused a reaction with the gelcoat. Under the blisters the gel coat looks "crazed", like paint after paint stripper's been on it. See photo:
surface flaw 1.jpg
To fix this- Do I sand all the old filler and gel coat off, and use epoxy based resin with chopped strands and/or silica filler or tissue to repair the surface. Or what.At first I thought it was osmosis... but not now I see this....


Luke Skywalker
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The first thing I'd do is try to determine whether what is left is stable. From whatI can see it appears as though the underlying laminate is solid. It also appears as though a considerable amount of fairing has been done in the past (Perhaps when it came out of the mold). A bit of sanding should show completely dry dust, nothing should stick together and nothing should flake off. If it seems solid, fill, fair and paint. Sand enough to provide a "tooth" for the filler. I would use an epoxy fairing compound with no glass fibers in it. That stuff is miserable to fair (it is usually harder than the surrounding gelcoat or filler). instead, lay a little piece of fiberglass cloth and unthickened resin in any deep holes that might need structural reinforcement. For fairing, epoxy with sanding filler mixed to peanut butter consistency works well for me. You can also buy pe-mixed filler.
I can't really see the crazing (cracks) from the picture but old gelcoats are prone to crazing and problems with the bond between the gel and the laminate. (Note that these cracks are in the gelcoat only. Cracks that go through the laminate must be cut out and patched.) Crazing can be easily covered with filler or even paint but the cracks tend to show through the surface after a short time and gradually worsen. The best fix is to remove the gelcoat completely then apply filler and paint but that is extremely time consuming. I find the faster methods of getcoat removal like peelers and media blasting too aggressive for thin car bodies. I compromise by sanding as much as I dare, cutting hollows into the bigger cracks, fill, fair, paint and then hope for the best.
Thanks for that.I've used a Dremel tool with a cutter to grind the cracks out into "V"s and filled them with epoxy/silica filler that I've then sanded down. The plan was to deal with the dozens of cracks that were fairly common and widespread and then deal with the blisters.These blisters are only on the very front and left door, so I suspect there may be accident damage that was covered up with a thin coat of filler that reacted with the underlying fibergalss. So... all that old stuff will have to come off and an epoxy filler go on.I plan on using a heavy grit orbital sander to take it down to solid material and then build it up from that with the filler. I've got silica powder to mix up with the epoxy so that's do-able.


Luke Skywalker
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Sounds like a good plan to me. However, I will repeat my pitch in favor of lightweight (marine?) fairing filler, at least for the final skim coat. While silica makes a very good filler material, it does not sand easily and you should not use polyester auto body filler over the epoxy. Tom


Darth Vader
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Go to one of your Scottish boatyards, show them a picture and I'm sure they will fix you up quickly.


Staff member
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Go to one of your Scottish boatyards, show them a picture and I'm sure they will fix you up quickly.

Nae! Haggis does nae stick tae fibreglas - nor oatmeal!
So... an update on what I've done these past several months. I ground out all the cracks and filled them in with filler, sanded those smooth and fixed a few little body flaws. I then stripped all the gel coat from the front half of the body, both doors and the roof, down to the fiberglass itself. Then coated it with two coats of epoxy resin. I'm now in the process of smoothing the surfaces a panel at a time. Hope to finish for Christmas with it primed. Need to get it running again too, but that should be easy by comparison, charge the battery and re-install the fuel tanks...(but that's another story).


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James, depending on how old the fiberglass is could determine just how long the repairs will last before hairline cracks start to reappear. The 1953 and 54 Corvettes fiberglass bodies were of a poor glass and after a few years would get hairline cracks all over, called cob webbing, because that's what they looked like. Many attempts were made over the years to repair these bodies, but the final solution was new aftermarket identical bodies were made from modern materials. They were so perfect that the Corvette club excepted them as original. What I'm trying to say is, don't be alarmed if in a couple years the hair lines start coming back, especially if the car is exposed to a lot of sun. Hopefully not! PJ


Freshman Member
I did the dreml out cracks, fill n paint on a mg hardtop a few years ago. Now there are new cracks. If I do it again I'll strip paint and sand off all the old gel and give it a coat of resin


Freshman Member
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In my experience if you really want to fix cracks you need new mat and tissue under the resin. It's a horrible job but the only way to a sound repair. Without new fibres things will flex and re-crack. I've previously used a disk sander to grind a shallow hollow. then layered in mat and tissue with new resin. The mat is strength and the tissue stops the strands from the mat showing in the final surface. Those repairs lasted over 10 years with no re-cracking of the problem areas.
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