Fiberglass panels spider cracks, stress cracks

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PostPosted: Tue Nov 08, 2016 7:02 pm
I had removed my wide fiberglass kit from my car that has been on it for about a decade as i wanted to redo everything as things have started to chip off

Well after sanding and inspecting i noticed these cracks on all 6 pieces of the kit, i was told by a shop that these would all need to be repaired before installing the kit and molding it to the car as they would show after the car is primered

I wanted to find out if that was indeed the case and if it is, how i would go about repairing them
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PostPosted: Tue Nov 08, 2016 11:16 pm
We see this a lot on "aged" kit cars. The first thing you have to answer is....What's causing this? With body kits that are fairly large pieces like yours most of the cracking comes from "flexing" of the panels, which can mean that there just isn't enough thickness to the panels for rigidity or they are just not supported enough by attachment. Soooo....fixing them is not too hard, but keeping them from coming back can be a real puzzler. Fixing involves taking a Dremel tool and grinding the cracks open to a v shape and making sure you go deep enough to get to the bottom of the crack. Some guys also ( I do this) like to drill a small hole at the end of each of those cracks which acts as a "stop" to keep them from running further and trying to connect. Once opened up fiberglass resin and finely chopped up mat can be fed into the areas. Epoxy resin is also an option for those as well. Once I've got them filled out I still like to "thicken" those areas with another layer or two of resin/mat on both the inside and outside of the panel. On the outside I continue by creaming over that whole area and feathering back in for my shape.
I tell you something I think I would have done before removing those panels was to have someone video tape the car while it was moving at say, 35, 55, and 75 m.p.h. You can actually see panels at least vibrating, or worse, flapping. I've helped guys do this before and then we carefully went over the video and started putting pieces of blue tape where there were flex problems. In almost every car a simple added brace here or there got rid of almost all the shake. I had a about a 1/2 inch shake causing cracking on one fender on a car I built. We were stumped. Finally had a company that did "on car" wheel balancing and they found the wheel was out of round combined with irregular shape in that corner's brake rotor. New rim, new rotor, no more shake, no more cracks.
I think the best thing you could do is still fix everything and remount the body still in primer and do the drive/test thing with it before moving on to painting. Might save you some headaches down the road with reappearing cracks.
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PostPosted: Wed Nov 09, 2016 5:07 pm
Well thanks for making me aware of not only fixing but prevention :)

So originally the kit was held on my rivets and bondo, they really didnt secure them well and everything started to crack and peel, could the presence of the rivets contributed to the cracking as everything flexed?

I have the 105 epoxy that i would use to mount them, my plan was to hold the kit in with self taping screws and after some areas are bonded remove the screws and fill in the holes so as to prevent any rust, i was going to use rivets but screws would be simpler to remove

Would that be enough to support the panels since im using a high quality resin to actually bond everything?

So after grinding to a V i should take some mat and and basically get it to a point of where i can sprinkle in the mat along with the epoxy?

My only problem is i have several of these panels and there are probably a hundred cracks total, some are pretty deep into the fiberglass

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PostPosted: Wed Nov 09, 2016 5:48 pm
In my opinion rivets are a huge no/no when it comes to "holding" these types of panels in place and well, bondo.... uhhh that is a filler only with little to no bonding strength at all. I'm almost positive that the rivets greatly contributed to the problem.

You already have the correct idea with using those screws temporarily and bonding with that West System 105. I love that stuff, we use it with the metered pumps for our wood restoration and I've built many body kits using it. Yes, it will provide a strong bond as long as you rough up the fiberglass and the metal to an 80 grit surface. Rough surfaces increase surface profile for more grab. There's a lot of old Datsun 240 Z kits still running around with the entire rear Ferrari clip held on by that West 105.

Yeah, just take a sheet of mat and pull some single layer hits of it. Take a sharp pair of scissors and chop it up into 1/8 to 1/2 inch strands to feed in with the resin. I batter the area with the epoxy resin and just keep feed the mat into the area making sure it all thoroughly wets out.
Yep, several hundred cracks... that sucks but you've got to take care of them or your new paint job will just be a real waste of time and money. Get a neighbor or friend to help you, it goes a lot faster. One guy buzzing and prepping with the other guy going behind and doing the fill/mat work.
Metal, wood, fiberglass, we work it all... www.furniturephysicians.com We can restore the irreplaceable!



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PostPosted: Wed Nov 09, 2016 9:05 pm
Yes i stay away from bondo esp since it allows water through

I called a guy that did work on corvettes and said that instead of repairing the cracks i could put a few layers of filler or fiberglass mat and epoxy over my panels to basically making my panels thicker and with the new layers the cracks would not go through

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PostPosted: Wed Nov 09, 2016 11:16 pm
The problem with that is.... Corvette guys are not working with ordinary fiberglass. They work with SMC fiberglass which comes out of pressure molds. Any Vette made after about 1973 has this pressurized fiberglass material and it will respond much better to that way of repair then what you have which is open lay up molded fiberglass. You can't just lay more fiberglass on top of them or they will simply "ghost" right back up through the new layers. One of the local "ricer" club guys just did this reconditioning his nose and rear bumpers in that manner. Car has been done for about 2 months and you can see the stress cracks coming back up through the paint.
Metal, wood, fiberglass, we work it all... www.furniturephysicians.com We can restore the irreplaceable!

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