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Psionic 01-06-2004 09:31 PM

Yet another question about bonding fiberglass to metal
First off, I'm making custom widened quarter panels for my car. Its not just a fender flare, its a little more complicated. About 1/2 of the panel will be custom, and the other 1/2 will be stock.

I can't remake the entire quarter panel out of fiberglass, that would be an insane amount of work, and I would never get it right.

I want to make the widened part of the quarter panel out of fiberglass, and mold that to the rest of the metal body. I want to use fiberglass because of the weight savings, its ease of working with, and the fact that it will never rust.

I've read every single thread on this board about bonding fiberglass to metal, some things were mentioned, but nobody really came to a solid conclusion.

I know that fiberglass and metal expand and contract at different rates, so you can't just slap one on the other an expect it to last.

But I do want it to last, so is there some type of buffer material that i can place between the fiberglass and the metal, like some kind of malleable adhesive to allow the glass to move around a bit?

Also, I hear there is some stuff that you can mix in with your paint so that the paint will not completely dry, to make the paint more flexible. Could I use this ontop of where the metal meets the glass (where the cracking is going to be) so that when the glass does move around, it wont crack the paint?

Is this ever going to work or am I just going to have to bite the bullet and rig up something ugly with metal? Please be open minded, I really want this to work.

troy-curt 01-06-2004 11:39 PM

Just use sheet metal \, the fiberglass will not work.
Weld it up grind it down and bondo it.


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Dubz 01-07-2004 01:01 AM

darn i glassed on my metal fabbed cowl's gonna crack, that sucks :(

302 Z28 01-07-2004 05:10 AM

It is really impossible to obtain a permanent bond to metal with fiberglass. One of the arguments made by builders of fiberglass cars using wood reinforcing is that very point. Wood will absorb the resin and will form a much more permanent bond. Metal will absorb no resin and the bond is on the surface only.


Psionic 01-07-2004 07:44 AM

Im not looking for blanket statements like "it wont work just use metal" here, I want people to think outside the box.


Originally posted by 302/Z28
It is really impossible to obtain a permanent bond to metal with fiberglass.
Metal will absorb no resin and the bond is on the surface only.


Thats why I asked if there is a buffer material that can be placed inbetween the glass and the metal, to allow for expansion.

So again, is there a material that can be placed between the metal and the glass?

What if i did not fasten them together with an adhesive, and did this:
1) drill holes in the glass where I want it to attach to the metal approx 1/4" in diameter.
2) drill holes in the same spots on the metal, appox 1/8" in diameter.
3) line the glass up to the metal, and feed a bolt through each hole.

The thread of the bolts would be 1/8" in diameter, so they would be snug in the metal hole, but there would be room for expansion in the fiberglass hole since they are 1/4" in diameter.

4) attach a nut to the other end of the bolt, but have some kind of gasket between the nut and the glass to allow things to move around a bit.

Maybe I could use the process above in conjunction with an adhesive.

If the above process will not work, please do not just say "it will not work, you are crazy", please tell my why it wont work and give suggestions on how to make it work. 01-07-2004 08:16 AM

You are getting blanket statements that it won't work because it won't work! Sorry. The only way I would try that is to build the fender flare, then encapsulate the entire fender in a couple of layers of 'glass mat & resin so the outer surface will be all one material. This would give you a fighting chance.

Psionic 01-07-2004 08:20 AM

So why won't the bolt/spacer method work? (the process I described above)

troy-curt 01-07-2004 08:33 AM

If it wont work why waste the time to explain? That wont make it work, It's been tried many times.

Once in a while you might make a joint that don't crack, but it wont last long.

Troy 01-07-2004 08:49 AM

You seem determined so give it a try. You may come up with the magic bullet we all missed. All we are telling you is that we have tried all the things we can think of and IT DON'T WORK!!!!!! :D :drunk: :D

Psionic 01-07-2004 08:51 AM

Then how does anybody make fender flares?
Just by banging them out of steel?

I was thinking I could get some pipe, bend it to the contour of my wheel well, and then cut a 1/4 of the curvature of the pipe out and weld that to the quarter panel.
But my flares are going to be a 3" radius, which means I would need a 6" pipe. I've never used a pipe bender before, can you bend 6" pipe? 01-07-2004 09:28 AM

Either do the steel body work or take a fiberglass mold off a mockup and make a fiberglass version. Either works great by itself, the problem comes in mixing the two. I don't have a mind's eye view of what you want to do. Can you cut some radial slits in the fender opening, form the resulting tabs to the bulge then weld in pie shaped wedges of sheet metal to fill the resulting gaps? That should be fairly easy to do and with a little Bondo, you will have a stable fender.

Psionic 01-07-2004 10:19 AM

1 Attachment(s)
I'm not a very good welder so welding little patches together might not turn out very well.

Anyway, here is a little concept drawing to give you an idea of what i want to do. I bought different tail lights that are going to fit right on there, so yes I'm cutting the metal where the original tail light is.

Call me stupid, but I think I'm going to go for the fiberglass / metal frankenstien. Im going to do alot of research and experimenting though before i take the plunge.

When we are taking about expansion and contraction of the metal, can you give me some idea of how much it expands and contracts relative to the glass? Are we talking like a 1/4 inch here or thousandths of an inch? 01-07-2004 10:52 AM

FRP coefficient of thermal expansion is about half that of steel (0.0000044"/"/F vs. 0.0000084"/"/F). Thus assuming you install the 'glass @ 70F and the panel heats to 140F in service (hot summer sun) and the longest joint in your setup is 36", then differential expansion will be about 0.010". 1/100th inch is plenty to overstress the bond between polyester resin and steel giving you a nice, long crack, regardless of the mechanical aids you incorporate.

Your design is pretty involved and will require some skill to pull off even in fiberglass. I suggest you take a welding course @ your local junior college and develop the skill to MIG weld sheet metal. It is a really easy skill to learn. Then you can do the job right and you will have a skill that will last a life time and that is one of the great side benefits of do-it-yourself hot rodding.

I would do that job using exhaust tubing. Someone out there surely does bends in 3" tubing. A good source to investigate is exhaust system suppliers for big diesel trucks.

Psionic 01-07-2004 11:15 AM

Actually I would have to find a person that can bend 6" tubing, because its 3" quarter round, meaning the original pipe would have to be 6".

Anyway, good info. Thanks. 01-07-2004 11:20 AM

Check the truck guys anyway. They make some pretty big exhaust pipes for them.

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