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Discussion Starter #1 (Edited)
I was wondering has anyone ever used gelcoat as body filler? The only reason I ask is that I still have a quart of gelcoat leftover from a project. It says it can be used over metal but I didnt know if it would stand up over time. I want to use it to level some areas no more than an 1/8 inch deep.
 

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do it right the first time

It never pays to cut corners or cut cost. Fill it with body filler and then use a high build two part urethane primer, spray some flat black over it, then use a long block and sand until the black is gone. Switch up on your directional sanding somewhat like an x and it will be perfect.
 

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If by gelcoat, you mean polyester resin catalyzed with MEK, I've had a good amount of experience with it. If you mean epoxy, I've never touched it but I can tell you the practice is much the same as the poly.

The fiberglass kit car guys go wild with it in ways us metal oriented guys couldn't dream of. The problem I see is it not sticking to steel. But that might just be a false impression. I think when you see a steel fastener embedded in a glass gelcoat, it's held there mechanically. If you're trying to build a thin gelcoat on a steel surface, I just don't think it would work, because the chemical adhesion is not there(IIRCC I could just pick the resin off my razor blades quite easily). Probably having something to do with bonds...ionic...covalent..who knows. You see when you apply the resin to a roughed up resin surface, it immediately turns clear...signifying a nice chemical bond. For an actual answer, I'd ask a Chemistry professor or Chem major.
 

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Gel-coat has no adhesive quality.

Metal will act like a mold and release the gel coat.

Fiberglass resin is what bonds to gel-coat, protecting the glass from moisture.
 

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I add a little gelcoat to my filler if I need it a little thinner, like glaze, for filling pinholes & for an extra thin spread. You can't use it straight because it has a tacky surface for glass to stick to, you can add special waxto it for top coating to stop the tackyness, but you don't want waxes or silicones in bodywork. It works really good in Duraglass & tiger hair for spreadability, otherwise it's like trying to stir & spread a dead rat on your project.
 

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Discussion Starter #6 (Edited)
I add a little gelcoat to my filler if I need it a little thinner, like glaze, for filling pinholes & for an extra thin spread. You can't use it straight because it has a tacky surface for glass to stick to, you can add special waxto it for top coating to stop the tackyness, but you don't want waxes or silicones in bodywork. It works really good in Duraglass & tiger hair for spreadability, otherwise it's like trying to stir & spread a dead rat on your project.
This is what I was looking for. I was looking for something with more spreadabily since Im trying to use it for smoothing. I just never thought of mixing the two.

So if Im understanding you correctly I should be ok with doing this. This will save me some time since I have both items on hand.
 

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If you are having problems getting filler to spread smoothly you have old filler, cheapo filler, you are mixing it wrong, or you are using it for the wrong application.

Why would you want to mix products together when you don't know the results?
It is a crutch for using the proper materials and doing things right.

Do you really want to take a chance that it may fail after you have gone to all the work and expense of painting the car? You'd be better of disposing (properly) of the extra gelcoat.

It has already been said that it doesn't bond well to metal, adding it to other products will reduce the bonding of the new concoction you have created.

I also think gelcoat would be much harder to sand than filler.

You will find that in any bodywork project there are plenty of things that can go wrong. Why not eliminate as many variables as possible by using the proper materials correctly?
 

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Yes, following the directions to a T will get known results. I've just been lucky enough to know alot of people in the fiberglass industry to learn alot of tips & tricks that are used everyday that aren't on the books. Too much gel in bondo will make it tacky & plug your sand paper, but just the right amount works in a pinch when you need it. I've done it occasion for 15 years & never had a problem or a comeback on a repair. I would have never done it, or the many other tricks if I hadn't seen reputable people doing it, and the results they got. If I have the money for glaze, I buy it. If I know I can squeak by with what I have on hand, I'll do my best to squeak by.
 

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Discussion Starter #10
I dont think anything is wrong with the leftover products I have. Ive used them all many times and know what results they provide. Im basically trying to fill very shallow imperfections and pinholes. I may look into different types of glaze before I even try this. I may hit a body shop to see what what kind of glaze I can pick up.
 

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There is a way to use gelcoat on steel successfully, sand the bare metal well with 80 grit, clean it well, apply two coats of quality epoxy primer and let it setup overnight, the next day you can apply any polyester product you like and it'll bite in good. The epoxy acts as a tie coat and the proceedure can also be used for repairing smc and carbon fiber products with polyesters-done it many times with never any failures.

I know of a few bodyshops that have used gelcoat for thinning fillers for years, it works. When I did it years ago I would let the filler cure up good then scrub off the tacky surface with a scotchbrite and lacquer thinner then it sanded fine. But fillers have come a long way and with the poly glazes and poly primers we have now there's really no sense in cocktailing your filler with gelcoat anymore.
 
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