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Primer on Fiberglass Fix?

mfleming

Well Known Member
Patron
Recently I sanded and primed my cowling with a 2K epoxy primer. Apparently I was over aggressive when sanding. The fiberglass cloth weave is showing on a fair amount of the cowl.

My paint shop told me "rough sand the fiberglass and apply a 2K primer and they will make it pretty". So, really the primer is acting as a sealer until it gets to the paint shop. Protection from oil and dirt....

My concern is, do I need to do something to preserve the integrity of the primer as a sealer or is it protected from contamination as is?
 

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You were not ready for prime. Much more body work needs to happen. I suggest you work it out with your paint shop on who will do what. If they are doing the body work they will again sand. I suspect you can just light sand and shoot another coat or two of primer if you have bare spots.

Not familiar with 2K primer. Is it UV resistant?

Carl
 
Recently I sanded and primed my cowling with a 2K epoxy primer. Apparently I was over aggressive when sanding. The fiberglass cloth weave is showing on a fair amount of the cowl.

My paint shop told me "rough sand the fiberglass and apply a 2K primer and they will make it pretty". So, really the primer is acting as a sealer until it gets to the paint shop. Protection from oil and dirt....

My concern is, do I need to do something to preserve the integrity of the primer as a sealer or is it protected from contamination as is?

I have a lot of pinholes when I primed the fiberglass parts. I was successful at using the high build primer which is layed up thicker than the regular epoxy primer. Then you sand down most of it away. The high build primer covers up all of the tiny pinholes and sanding imperfections. Then you can spray epoxy primer on top of the surface to seal it or just spray paint on top.
 
You were not ready for prime. Much more body work needs to happen. I suggest you work it out with your paint shop on who will do what. If they are doing the body work they will again sand. I suspect you can just light sand and shoot another coat or two of primer if you have bare spots.

Not familiar with 2K primer. Is it UV resistant?

Carl

You're very correct about not being ready for primer as far as final paint goes. This primer is really just protection from oil and dirt until the AC reaches the paint shop.
 
The surface in the photos appears porous so contamination will likely get in.
It doesn't look like the primer has sealed because the surface was too rough for primer.
Probably a brushed on coat of epoxy resin would've been a better sealer if the shop is going to do the bodywork filling and block sanding.
As to the fix now, like Carl says it would be best to talk to the paint shop but I think it will need filling before sealing.
 
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No worries with the primer but now there is work to be done. What you can see now is where every pinhole and void is located. Fill them, sand it smooth and prime again. If there are no pinholes this time, sand it smooth and paint. If there are pinholes, fill again and prime. Sand it smooth and paint.

A complete set of fiberglass parts takes me 2.5 days start to finish.
 
Fiberglass prep

I am a hack, but I get really lucky.
Dan Horton method with minor deviations. I spread micro, sand flat then a coat of neat epoxy. Sand that flat then two brushed coats of 2K epoxy primer. Wet sand that flat and there's almost nothing left to seal. The parts look like heck but they are smooth as a baby's bottom. At that point I shoot primer. If there are any imperfections, they get a tiny smush of glazing before the final paint.

Honestly, I think it's fine if you patch the area with some 2K primer. That's what the paint shop recommended. They will sand it anyway.
Again, I know nothing!
 
What Carl said. The surface was nowhere near ready for an epoxy or urethane primer.

I'd sand off all the primer, back to bare glass, roll or brush a coat of System Three ClearCoat (or a similar water viscosity pure epoxy) to fill and seal, scuff sand when cured (without breaking through!), then shoot epoxy primer followed by a high build primer as soon as the epoxy primer's solvents flash off. Block sand for flatness.
 
I appreciate everyones input.

The reason for applying the epoxy primer is not for final paint...it's sole function is to protect the fiberglass from oil and dirt until it makes it to the painters...they'll sand it off and do whatever it takes to make it pretty.

My question was/is, do you think what's on there will protect it from oil/dirt?

And since my original post, I sprayed another coat of primer...
 
What Carl said. The surface was nowhere near ready for an epoxy or urethane primer.

I'd sand off all the primer, back to bare glass, roll or brush a coat of System Three ClearCoat (or a similar water viscosity pure epoxy) to fill and seal, scuff sand when cured (without breaking through!), then shoot epoxy primer followed by a high build primer as soon as the epoxy primer's solvents flash off. Block sand for flatness.

No reason to back to bare unless the the surface was never sanded.
 
I appreciate everyones input.

The reason for applying the epoxy primer is not for final paint...it's sole function is to protect the fiberglass from oil and dirt until it makes it to the painters...they'll sand it off and do whatever it takes to make it pretty.

My question was/is, do you think what's on there will protect it from oil/dirt?

And since my original post, I sprayed another coat of primer...

Do I think what's on there will protect from oil/dirt? Nope, because there are visible voids that can trap oil/dirt.

How long will you be flying it before your paint slot, i.e. not at all; phase 1; 12 months? That may influence how much protection you need.
 
No reason to back to bare unless the the surface was never sanded.
The surface was completely sand...too much so in some areas.


Do I think what's on there will protect from oil/dirt? Nope, because there are visible voids that can trap oil/dirt.

How long will you be flying it before your paint slot, i.e. not at all; phase 1; 12 months? That may influence how much protection you need.

The hope is that I can get it into the paint shop shortly after it flies...

Since the photos, I added a second coat...
 
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Popcorn

Finished my popcorn. Making a new batch! :D
Like the man said, just trying to protect it till the paint shop.
Epoxy fiberglass needs to be protected from UV.
Enjoying my popcorn.
 
No reason to back to bare unless the the surface was never sanded.

The advice was to return to bare glass and shell it with epoxy, the result being a fully sealed surface...the goal here. Subsequent primer would largely be for UV protection until it can be test flown and taken to the paint shop, which seems to be the plan.

I don't think Mike is going to do it, given he is spraying more primer, so what process do you propose, and why?
 
The advice was to return to bare glass and shell it with epoxy, the result being a fully sealed surface...the goal here. Subsequent primer would largely be for UV protection until it can be test flown and taken to the paint shop, which seems to be the plan.

I don't think Mike is going to do it, given he is spraying more primer, so what process do you propose, and why?

I sprayed a second coat of primer because I’m losing my window to spray outside and I wanted to do something quick.

For what it’s worth, I agree that sanding down to glass and shell with epoxy is the best option. It’s what I did on the windshield fairing and canopy skirt.

In hindsight, I should have taken the time to do that before priming. I thought it wasn’t necessary because the paint shop was going to do their magic on the cowling and all I had to do was protect it from oil and dirt.
 
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UPDATE: I sent current photos of the cowl to the paint shop and they were comfortable that the cowling is protected until they get their hands on it.
 
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