Why epoxy primer?

[beedees]

Will aluminum still need etching primer?

[JB400]

Are you wanting the paint to stick to aluminum?  If so, then yes

[Silver R/T]

You can epoxy over aluminum, no etching primer needed.

[beedees]

I guess I should have asked, what are the benefits of using an epoxy primer over anything?  Better adhesion, moisture resistance or just the mfg. bottom line?

[Brian in GA]

Epoxy has several advantages over standard primers. It's more expensive, but first off, it is non-porous when it cures, which means that it won't soak up moisture like conventional primers and primer sealers do. Because it's waterproof, it acts as a sealer over the metal as well as denying oxygen and water to the metal, resisting rust. 

The other advantage is epoxy's adhesion abilities. Its chemical makeup provides excellent adhesion to the bare metal surface, making it the ideal base coat for all the necessary primers, sealers and topcoats to follow, as well as any body filler.

Not to mention you actually get better adhesion of the filler on top of the epoxy coat than you would straight on to bare metal. 

That's just my two cents. 

[beedees]

Is epoxy for use only on bare metal, or? 

[bull]

Yes, bare metal. However, I don't see much advantage to epoxy primer on aluminum, especially given the way it has to be used. The main benefits of epoxy are generally aimed at protecting steel from the things that cause rust. Aluminum obviously doesn't rust. But I would still use some kind of primer and I would scuff the aluminum before priming it.

[Silver R/T]

Yes, bare metal. However, I don't see much advantage to epoxy primer on aluminum, especially given the way it has to be used. The main benefits of epoxy are generally aimed at protecting steel from the things that cause rust. Aluminum obviously doesn't rust. But I would still use some kind of primer and I would scuff the aluminum before priming it.

Advantage of using epoxy over aluminum-adhesion. You wouldn't simply scuff and paint your aluminum hood, etc. It has to be sealed and that;s where epoxy comes in. You can reduce most epoxys up to 10% for that sealer coat just before paint.
Other benefits of epoxy, as a sealer it will minimize body filler bleed through or darker color bleed through if you're going with lighter color and need to seal off underlying paint. Better adhesion, moisture/air barrier.

[bull]

Well, I guess the extent of fuss over it would depend on whether he's painting a hood for a show car or a bird house. But according to the advice I've seen, some etching needs to take place, be it manual or chemical.

[Brian in GA]

Bull, like you said, depends on what he's painting.  As far as the etching, scuff the metal with 90 grit and spray away.

[Silver R/T]

I would recommend SPI epoxy primer
http://www.southernpolyurethanes.com/Product%20lines.htm
http://www.southernpolyurethanes.com/TechManual.pdf

[Brian in GA]

I would recommend SPI epoxy primer
http://www.southernpolyurethanes.com/Product%20lines.htm
http://www.southernpolyurethanes.com/TechManual.pdf

That's exactly what I use.  Good folks to deal with and will answer any questions you can come up with about painting.