What grit to use between epoxy primer & bondo?

[coclimber]

Hi!

I sprayed two coats of epoxy primer before putting on Bondo. Then, according to the Bondo instructions, I VERY lightly sanded with 80 grit. However, now I swear I see shiny metal showing through , which is not what I want!! I want it sealed with epoxy before putting on body filler.

Do I need to still use 80 grit on the epoxy or is that just the Bondo instructions for bare metal adhesion requirements? If I scuff it up with say 320, will the Bondo adhere OK to the epoxy?

Jason
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'83.5 Westfalia Camper - SOLD
'86 Westfalia Weekender
2001 Subaru Legacy 2.5L donor
Rocky Mountain Westy kit

[jspbtown]

First of all I assume you are not using "Bondo" brand right?

The more "tooth" the better for fillers. I would just go ahead and apply to what you have. Seal it again with a coat of epoxy afterwards.

[coclimber]

[NewTechnicIan]

I just got my bug in paint, so longevity still needs to be tested, but if you're seeing some metal through the epoxy after sanding, I wouldn't be too worried. Filler likes to be applied to tooth, whether that's in epoxy primer or in metal. Both are waterproof, and you're going to be putting epoxy/high build over the bondo afterwards anyway. I originally scuffed a few panels with 400 grit then found out that was a BAD idea, had to wire wheel it all off, rescuff with 80 grit and apply filler over that.

Hope that helps.
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My 73 Standard build - It's painted! It drives under its own power! Time to get it wired and upholstered!

My 1679 Engine build.

[nemobuscaptain]

[kman]

Often it is not required to scuff or sand epoxy before using filler if the filler is applied within a couple of days. It chemically bonds. Check the Technical Data Sheet (search online) for your specific primer to be sure. If unsure, anything 150 or courser is enough tooth for filler. Filler adheres to bare metal well also so I wouldn't worry about a little showing here and there. If you are sanding through right away your coat must be pretty light in the 1st place.

[Julio_Arcos123]

Many people will say dont put filler over primer/paint. Different companies will say different things. Its preference really. Some swear it will peel, i personally have never had problems with it.

80 grit is just fine. 36 will just remove everything and dig into the metal, how it should be put on. 80 whether by hand or d.a. will give it just enough scratch to where the filler will hold on and grab to. 180 or lower will basically only scuff the substrate, here is where you get risky. It will stick but i wouldnt trust it, specially if youre throwing on a lot.

With 80 you might get to metal but as long as you put your filler then primer before surface rust builds,or it is exposed to moisture, you are all good.

Again the more "teeth" you have for your filler to hold on to the better.
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64 bug (o\ ! /o)

[jason]

Epoxy has a window to put filler on top of, a week for my SPI. It will stick nicely. I was also under the impression filler isn't waterproof. It has pin holes.

[jspbtown]

Filler is not waterproof. But you coat it with high build, sealer, top coat and clear.

[jason]

That's why I like the epoxy not sanded or ground down. Keeps the moisture out. Paint doesn't keep moisture out from what I've heard. That's why paint shops have to put clear on, no more single stage. The paint releases into the atmosphere, if it comes out it can go in. Clear will seal it but when the clear starts to come off, well you know.