sonnyhumble,
I think you are still missing the whole point. Abarrent is not suggesting going to the shelf and picking up a can of "sealer", he is talking about putting a layer of epoxy primer which happens to be an incredibly good sealer, over the sanded car before starting any repair work. That gives you a solid, dependable base to work with. Then filler, hi-build primer, and last, a true dedicated sealer.

I understand.. but are'nt epoxy primers difficult to sand.. I mean.. their not supposed to be heavily sanded..atleast the ones I use.. I use MP series Epoxy Primers from Matrix Systems as a sealer before topcoat, it can be sanded.. but very lightly only with somthing like 600 or higher.

Yeah, epoxy primer as a base, just scuff it before more paint if the recoat window has expired. Use a conventional sealer or primer/sealer over all before colorcoat. 2 different products for 2 different stages of the project.
We are, of course, referring to a high quality job here.

ok..so I am confused then.. I mean.. I understand what u just said completely.. but why add epoxy primer/sealer over the whole car BEFORE repairs are made?? epoxy is not really a workable /sandable product.. I understand useing it before color coating.. but not before repairs of dents and whatnot

I think maybe we're really arguing semantics here... when I say "sealer" I mean a coating that serves primarily as a sealer, which means increase topcoat coverage, seal from bleeding and moisture, and improved adhesion. Even though my epoxy can says "sealer" on it, it might not be what you're thinking of as "sealer", as in a coating meant to be applied over a primer for topcoat adhesion, coverage, and sealing.

As for the benifits of epoxy and when to use it... theres so many. You can use it underneath a topcoat for increased adhesion and get all the benifits of a "sealer" sealer. It sticks to anything so I use it as my first coat on any substrate just as insurance. You're right epoxy isn't sandable, and you don't want to use it before ANY repairs are made, especially any repairs that require metalwork. Once the metalwork is done, the reason to use epoxy is to increase adhesion to the metal for subsequent coats, and to seal the metal against moisture, which of course causes rust. With bondo this is especially important because the resin it uses is permeable. In otherwords, bondo allows moisture through it, and if the bondo touches metal then the moisture can reach metal and rust. To make it worse, the filler component is talc, which sucks up water and doesn't let it go. Just a tiny pinholes is all the exposure a chunk of bondo needs, and given some time the talc will suck the water up until it's near saturated, and it'll hold the moisture up against the metal. This is an exaggeration, it isn't dripping wet, but it does happen. If there epoxy under the bondo then no real harm is done to the metal and no rust.

This is why bondo sucks and part of the reason why no quality repair will use the crap. The best filler material would arguably be either metal filler (lead or whatever, old school but quality), or epoxy resin (not permeable) with powdered aluminium as the filler component. This has a similar thermal expansion to metal and it won't allow moisture through it.

I regard everything I just said as fact, not that epoxy is necessary, theres more than one way to skin a cat, but the basic theory behind everything I just said is the same theory that any qualified or competant painter or bodyman should be aware of when they prep metal for paint.

Hopefully that fills in anyone reading and scratching their heads as well as puts an end to the bondo on bare metal debate. Any argument for putting bondo to metal comes ont he basis of "it's not that bad", which is just an opinion on an already established fact: it's not the best way to work.

I thought I'd add just as I hit submit...

On that last line: It doesn't produce the highest QUALITY work, it might be the best way when you consider how easy and cheap it is to slap some mud over bare metal and sand it down like it was modelling clay.

Completely understood.. I work in a pro shop, we do 99% insurance jobs and we dont do alot of mud work.. mostly.. a bent up panel will be replaced, but on new panels even.. say for instance a hood.. some people just hit it with a scuf pad.. I dont do that.. I sand it good with some 320 on a da.. ofcourse you break thru to metal on new panels alot when u do that.. so I just shoot the epoxy primer/sealer over it and go to painting.
I use epoxy sealers everyday.
However, I do agree with what youa are saying about a better QUALITY job by apllying epoxy to bare metal before bondo. or before paint.

You also have to take into consideration.. that we are not rebuilding OLD cars and restorations.. we are rebuilding totals or close to it, so our body men still do an excellent job.. but I dont belive they apply epoxy before mud werk simply for the fact that the repair they made will probably out live the life of the vehicle.

We come from different worlds, not on the same page. I don't do production work, only high quality custom stuff, so to me putting bondo on metal is the stuff of nightmares.

I see what ya mean.. no harm taken.. though I dont do production work either.. my definition of production work is what you will recievce at Macco or SPray-glo.. the $199.00 paint job shops.. not us, we do QUALITY work, we accualy have the best reputation in town for high quality.
we mainly work on year 2000 on up.. so the work has to look like it was never performed, or the accident never took place.. the majority of the time.. you can never tell we made a repair or even painted a panel on the car.

That's cool, it would suck to work at maaco, it's depressing to never get the chance to be proud of your work and just churn out **** all day long. I always called any insurance or repair work production work, the alternative being custom work, which doesn't necessarily mean graphics, kandy and flames, but could just be a color change.