If you have already converted (phosphated) the bare metal then an epoxy primer would be a good choice. Most epoxy tech sheets show this as a preliminary step to applying the epoxy primer.
Epoxy primers come in different varieties and today's formulas are usually lead-free (LF) or chromate free. The absence of lead and zinc chromate in the filler materials used in epoxy and other primers is safer but lessens the effectiveness of the sealing capability and moisture/rust inhibiting quality of the product. There are still some good protective epoxies with good solids content available-but most of these are found in industrial or aircraft finishing products.
Rust/corrosion inhibitting does NOT mean rust nuetralizing or proofing. It means that it inhibits (slows down) the process of corrosion.
Lead and zinc chromate are added to primers and paints because of their dense and large molecular structure which helps make a barrier against moisture and UV thereby helping slow down rust and corrosion. Because of environmental and health concerns, fewer primers and paints contain these products.
If anyone has some good high solids epoxy primer information that they have good experience with, please add the name and company to this post. I have looked at tech sheets for some Western Automotive/Sherwin Williams finishes and Dupont epoxies in the medium build range. Phillips Industrial (PPG I think) glass filled epoxy is another interesting product I'd like to try.
Most of todays DP and other epoxy primers/sealers are lead free and sometimes chromate free and low in solids. They don't have the sealing and rust inhibiting qualities that were available in the original formulas of 20 to 30 years ago. 20 years ago PPG had better high solids epoxies in their aircraft line-DPU35 for one, and another I can't remember the part number for. These time frames are when putting body filler over epoxy primer started to evolve as a technique that was still not thouroughly proven as a surefire effective method of corrosion control for bare metal under filler. Also it was reccommended in the tech sheets for most epoxy primers, that bare metal be prepped with metal conditioner/treatment and cleaned prior to the epoxy being applied.
Epoxy does NOT nuetralize rust.
It will work to put body filler over epoxy primer and also most 2 part high solids catalyzed primers as long as the primer film has cured and released the solvents/thinners in it and is not affected by the styrene in the filler and the primer is scuffed/sanded for mechanical adhesion of the filler. Most of todays fillers/bondo have new resin technology that gives the adhesion a level comparable to epoxy glue. All body fillers and primers "stick" by mechanical adhesion. Self etching primers "stick" by mechanical and chemical adhesion. The phosphoric acid in etching primers can drastically slow the cure of body filler/polyester resin so this is not reccommended for use under filler.
Phosphoric acid will also affect the activator/hardner of some epoxies.
Etch primers are very good for what they were designed for and come in different application varieties.
1: Mild self etching primers with some solids and filling/sanding capability which can be used OVER body filler.
2: Vinyl wash primers with a more aggressive etch/acid content ratio and a VERY thin film build. These vinyl wash primers need to be primed and topcoated over and are mainly used instead of using a metal prep/phosphate coat and should NOT be used over body filler.
Always consult the manufacturers tech sheets. When using metal etch products (acid based) always use caution concerning the possibility of acid residues left in the film before applying something on top that could be affected by the acids such as:epoxy activator/hardner or polyester/fiberglass resins.
Sealers for application between the undercoats/primer and the topcoats/paint are usually limited in how effective they are at stopping solvent penetration, especially when the paint is applied wet on wet to the sealer. If you can take a rag with solvent from the paint and wipe off the fresh sealer coat even after 30 to 60 minutes, how effective are they at sealing. Most of these are really "tie coats" that help bond the 2 layers together. Most basecoat paints are actually better "sealers" when it comes to solvent penetration due to the parafin used to orient the metallics and pigments, but these will still let solvents thru. The best "sealing" is accomplished by good quality 2 part high solids primers or fully cured 2 part paint finishes. Tie coat sealers or epoxy sealers/primers are still a good part of a complete refinish system.