Can I fiberglass over epoxy primer?

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PostPosted: Thu Sep 05, 2019 7:48 pm
This may seem like an incredibly naive question, but I'm having trouble finding a straight answer online. I have a 1982 xplorer camper van with several leaks in the back and a rusty floor that I'm repairing. I should have fixed the leaks first but I already pulled some of the old fiberglass, got rid of the rust and have painted the floor with raptor epoxy primer. I'm just now realizing that I may not be able to fiberglass over the epoxy primer, but I've read that if you scuff the epoxy with 80 grit that you can fiberglass over it depending on the kind of fiberglass.. which kind I'm not sure. The kind I have is Bondo fiberglass resin. Does anyone know if this is possible? I was really hoping to be able to have the metal protected with the epoxy primer while I insulate the walls so the floor doesn't rust anymore.. and in order to insulate the walls I have to wait to close up the wall with the fiberglass after I'm done insulating. I really did things in the wrong order but I'm here now so I got to make the best of what I got. :knockout: Thanks!

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PostPosted: Fri Sep 06, 2019 7:36 am
Since bedliner is designed to repel just about everything I'd get it off there to apply your fiberglass. Nice, rough, shiny, metal is always a plus when applying any filler. Bondo fiberglass is okay for what you are doing but I'd upgrade to Evercoat products in the future.
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PostPosted: Fri Sep 06, 2019 12:21 pm
Have you applied the Raptor coating yet or just the Raptor epoxy primer?

If it is just the primer, you will need to find out what the chemical characteristics are and if a polyester resin is compatible with it or not. This may be difficult as it is part of a specific system and they may not give many details for any other use than as a prep for their top coating system.

Your "Bondo" resin is probably a Polyester Resin vs. a Epoxy Resin

Epoxy Resin Characteristics:
Epoxies are generally stronger than polyester resins
Epoxies are more expensive
Epoxies cure fully when applied as a thin film, which are good for adhesive applications.
Epoxies do not shrink when cured.
Epoxies are more resistant to wear, cracking, peeling, corrosion and damage from chemical and environmental degradation, which you may not need.
Epoxies are moisture resistant when cured.

Polyester Resin Characteristics:
The shelf life of polyester resin is about 6-12 months depending on how it is stored. I trash mine after 3 months.
You can vary the cure time of polyester resins by adding 1% to 2% of the MEKP catalyst.
Polyester resin generally costs slightly less than epoxy resin.
Polyester off-gases VOCs and has strong, flammable fumes.
Polyester resin does not cure as well as a thin film, so it doesn’t work as well as an adhesive.
The larger the pour, the more polyester resin shrinks when cured. This can be a huge issue.
Polyester resin is UV resistant and does NOT need to be top coated to prevent degradation from sunlight.
Once cured, polyester resin is water permeable and therefore needs to be top coated.


Polyester resin will not bond to epoxy resin. That does not mean that you cannot get a polyester resin to physically bond to an epoxy primer (different animal)

Don't purchase any fiberglass resins from the local big box store or auto parts shop. You don't know how long they have been sitting around. Find either a Marine supply shop and talk to the manager to make sure that their product is turned around on a regular basis or better yet find a plastic or fiberglass specialty shop. We have a local chain called TAP plastics.

You will pay a little more but end up with better service and a better product.
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