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Can you fiberglass over epoxy primer?

#1
I have an old 1982 Dodge xplorer camper van and have removed all the old fiberglass that was on the metal bed frame so that I could remove a ton of rust that was underneath. My plan was to clean the metal then spray epoxy primer to serve as a final coat and to protect metal from rusting while I spray foam insulation behind the walls, then seal it all up with fiberglass. So far I've already sprayed epoxy primer on 3/4 of the project area.. but i just learned online that apparently you can't fiberglass over paint?. Does this include Raptor epoxy primer? What I have is Bondo fiberglass resin which is polyester resin. Is there any chance that I don't have to scrape off all this expensive paint? Can't I just scuff the primer with 80 grit and fiberglass over it?? Thanks!
 
#2
anyplace you put that spray foam will rot in no time. it is like a giant sponge. you can use vpa as a bonding agent over spi epoxy if scuffed good with 100 grit .
 
#4
The spray foam is closed cell so I don't think it will absorb water.
The epoxy primer is u-pol raptor but I was told the best place to go for this question was SPI forum.
 
#5
i can tell you if i was to do this that i would epoxy primer really good, scuff with 40-80grit then layup the glass over it with epoxy resin. i wouldnt use polyester. i personally would have blasted the metal then put 3 or more heavy coats of the spi epoxy on it. i know this would have worked but i dont know anything about the raptor product you used so i cant say for sure how that would work.
 
#6
The spray foam is closed cell so I don't think it will absorb water.
The epoxy primer is u-pol raptor but I was told the best place to go for this question was SPI forum.
they claim that about foam it green. everywhere i used it on my metal building the sheets have rusted through . water runs through it.
 
#8
i can tell you if i was to do this that i would epoxy primer really good, scuff with 40-80grit then layup the glass over it with epoxy resin. i wouldnt use polyester. i personally would have blasted the metal then put 3 or more heavy coats of the spi epoxy on it. i know this would have worked but i dont know anything about the raptor product you used so i cant say for sure how that would work.
Thanks. I'm thinking I might get away with closing the gaps with some metal sheets I have and then sealing it with some sort of caulk. I already fiberglassed the area I hadn't painted yet so I hope that it works and doesn't end up popping off or anything at some point.
 

jlcustomz

evil painter
#10
Wonder about epoxy resins for fiberglass over spi epoxy?

Not fully related here, but I found a few extreme stress test samples I did in 2016 cleaning up at home last week. I sprayed spi epoxy on TEO bumper material (2016 camaro & vette ) & embedded fiberglass matt into the wet epoxy with plenty of loose dry matt to grab & pull on. Plan was to next apply epoxy resin & more matt which I never did.
Initial test several days later the epoxy would first pull off the TEO plastic with enough force, then the fiberglass matt fibers would somewhat come loose from the spi epoxy. 3 years later the epoxy was much more stuck to the TEO plastic & the embedded dry fiberglass fibers would just snap off from above the epoxy.
Original purpose for test was to see if I could bond fiberglass work to TEO bumper plastic for customizing. Had I gave the samples a few more weeks drying time back then, I may have went further with that plan.

Polyester fillers & resins really are 2 totally different characteristic materials.
 
#11
Seems odd that they want you to epoxy prime before using a polyester body filler, but cant use a polyester resin over it.
I should be clear, this is not a commission or anything, this is my personal project so I can travel cheaply. I have no experience with fiberglass or epoxy primer or.. anything car related really. Which is why I'm here asking what probably seems like stupid questions lol. But I'm determined to get this piece of junk I inherited in decent condition so I can hit the road!
 
#12
I should be clear, this is not a commission or anything, this is my personal project so I can travel cheaply. I have no experience with fiberglass or epoxy primer or.. anything car related really. Which is why I'm here asking what probably seems like stupid questions lol. But I'm determined to get this piece of junk I inherited in decent condition so I can hit the road!
I am the king of stupid around here, so dont feel bad. I just get confused easily. What you were doing made sense to me. I know there are certain fillers, like Metal2Metal that are supposed to go, on metal, but are told to prime first. I cant see why the change with the polyester filler.

Look, if you are epoxy priming before foaming, I dont know how the steel will rust out because of the foam. I get it if you are foaming on bare steel, but there is epoxy there. There are just times when things do not add up. We want epoxy down first before anything gets done, but then can use epoxy as a sealer on top of polyester.

I am not calling you stupid by any means, just thought you were good with what you were doing and would have gotten some life out if it.
 

Barry

Paint Fanatic
Staff member
#13
NEVER use fiberglass over epoxy.
The risks are too high and are all against you.

That is the same company that made us change our answer, as their fiber-all, that I used and other shops for years worked great, and then they changed something, and it would peel like a band-aid. About five batches later it started working again.
So flat answer from us is NO, as they are not going to tell us when they change something.
 
#14
NEVER use fiberglass over epoxy.
The risks are too high and are all against you.

That is the same company that made us change our answer, as their fiber-all, that I used and other shops for years worked great, and then they changed something, and it would peel like a band-aid. About five batches later it started working again.
So flat answer from us is NO, as they are not going to tell us when they change something.
Right, I forget that fiberglass is a brand, kind of like Kleenex. There's a tendency to use the big brand name as a synonym when that shouldn't be the case.
 
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