Don't feel bad. They never bothered to tell me about the lead (and other heavy metals) in ecoat when I was working in the industry. Ecoat is actually waterborne, and I used to think water = safe. I just wrote a lengthy note stating water not equal safe on another thread. I had weeks where my team would sand 4000-5000 panels in a week for exposure testing. I spent a lot time and money pumping the heavy metals out of my system from that experience.

Never sand non sand primer. Take a look at the MSDS, and you'll see lead, hexavalent chromium, and other fun stuff in there to keep steel from rusting. I would place a light coat of sanding primer (epoxy w/o the metals) over it to create a sanding surface.

In an auto plant, ecoat is never sanded (on purpose). You will acutally have better adhesion to the metal if you place the epoxy primer over the ecoat. There really isn't any subsitute for that stuff.

I would use the sanding primer over the body filler. That is what I used to do, and it worked well. you did not need to scuff it to get that to stick. The chemistries of good filler and epoxy primer are compatible. Now using the sanding primer under filler, I really do not know the answer. My technique was to fix the hole (after careful blasting) with Duraglas and fiberglass sheets applied to both sides of the hole, use the all metal to sand and smooth the surface, prime and paint, and renew undercoating on the other side if necessary. I never tried primer, then filler. My technique of sealing the hole never allowed water back into the area. No water = no rust. Never had a failure, even in the weather up here. Most of the filler contact was with the Duraglas patch. Never did have any failures where the filler feathered over the metal. I think you'd have to try the filler over the epoxy and see if the epoxy would support the filler. My worry wouldn't be adhesion. My worry would be splitting of the primercoat and that causing the patch to fall out. Definately do not spray too much primer on the area to be patched. I really don't think that this would be an issue unless too much primer is used.

So, what are the other fillers that like PanzerSD? I have done this type of work for years. There should be better materials out there now. And I agree: Bondo = Veeery bad. It is just too water soluble.

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2010 Toyota matrix

'93 500 SEL
A bad addiction. Takes all of my cash.

'12 Volvo S80 T6
Needed something that wasn't as hard to deal with as my bad addiction

'18 Mazda Miata
No more boring cars for everyday transport!