Thread: what is the right way to fix stress cracks in fiberglass

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I have an older fiberglass body made around 1993 and I am starting to get stress cracks in some places coming thru the paint . I am planning a repaint soon and would like to know before I take it to the body shop how this need to be fixed. The stress cracks are spiderweb shaped mostly around the A pillar and where the top transitions to the trunk on the side of the car. Should the cracked area just be ground out and filled with fiberglass or is there work to be done behind the crack first (reinforcing with fiber'gls mat) or am I in serious trouble with a body that has outlived its self proably cant see it in the picture but there little lines under the paint going from lower left in pic to upper right.

If it's an area you can get to the backside, it would be good to grind it down and add a couple layers of matt. For the outside, works good to use a die grinder to gouge out the cracked area and fill it with resind and small pieces of matt worked all the way into the crack...

I certainly wouldn't throw it away! :-) I have saved bodies that were totally covered with stress cracks. You need to not only repair the cracks, but add more stiffness.

I would do both things you have described. First I would see if I could do anything in that area to reduce the flexing, by addind a brace or something.

To repair the outer body, grind it back to glass and use a heavy grit, like 36 or 40. Rough the surface well, and make sure you have gone through any gel coat, and be sure to exposed the fibers in the glass. Work resin into the cracks, then lay up two layers of fiberglass "matting" over the damaged area. After it cures, put a few thin layers of plastic filler over the glass. This will make it cure completely, then block sand it flat, and primer heavily with catalyzed urethane.

On the inside,...to stiffen the area... I would repeat the process, but use two layers of fiberglass "cloth", and don't worry about the filler. It will cure with time.

Thanks guys thats what i needed to know . I knew you guys would have the answer.

If you are going to use matt on the inside, I have a roll of high strength matt that we use on the roof and floor of cars. It's quite thick and for appearance sake requires a layer of cloth over it. Let me know if you're interested in trying it.

These guys are right on, Mike. I might add that my glass body expert told me that is critical to do the inside work too, or the spiders will certainly return.

Originally Posted by Dave Severson

If you are going to use matt on the inside, I have a roll of high strength matt that we use on the roof and floor of cars. It's quite thick and for appearance sake requires a layer of cloth over it. Let me know if you're interested in trying it.

Dave I think its called Heavy Roving Mat , Used for greater reinforcing strength & Used on the bottom/under side and the glass fiber mat is used for top side repairs . I usually use a thin coat of Dura glass body filler over the top side of my glass repair , then a skim coat of Metal glaze polyester filler to fill the small pin holes that the dura glass leaves behind then a good 2k primer like HRP mentioned .

How much buildup should I do in the areas of question? I have some mat that is woven it is about 1/16" thick I would guess, would a couple 3 layers work or are we talkin 3/8" inch worth of buildup?

well alot of it to is to much gellcoat and not enough fiber i would lay up on the back side mat and use fiber gell works great over head i use to put busted semi truck hood s back to gether with this stuff

Mike , doing the under side repair I would grind back quite a bit , then layer about 3 layers one being a bit bigger on each over lap about an inch all around , and you might want to find a fiberglass roller to work out the air bubbles .

same with the top repair , grind back a ways from the cracks , then lightly V the crack area , then pull the fiberglass matting apart and dip it in your resin and using gloved hands put it in your V groove repair area , then cut a few thin strips and lay over the top , shouldnt need more then 2 layers over this repair . and then roll your fiberglass roller over it to smooth out air pockets .
Let set up and lightly shape with a 3" grinder . then use filler , sand and prime .

i would sand it with 36 but would not get nuts looks more to me a brake in the gell from fex then a break thru the mat but this would have to do with if it was a chopper gun body or a hand laid up body if chopper gun you may have to dig in deeper . for the small bit you have to do i would lay up carbon fiber mat