Epoxy primer and Etch primer is often debated on which product is superior in terms of corrosion protection, first we need to understand the difference
between the two
Etch primer also known as acid etch, self etching primer and wash primer and is a mixture of of acid and zinc phosphate, the acid helps by etching into the metal to provide a chemical
bond (adhesion) of top coats and the zinc phosphate provides corrosion resistance to
a certain degree. Substrates recommended are galvanised or normal steel, can also work well also on top of plastic and composites because of the acid but always check with manufacterer and TDS !!
Easy to use with minimal preparation, only thin film needed and its fast curing characteristics make it the ideal primer to use on top of any exposed bare metal on small repairs and quick turn over work.
Epoxy primer - A non-porous high build primer with excellent adhesion, can be used on top of bare metal, plastics, composites such as fibreglass and carbon fibre resins.
depending on the brand it can take from 24 hours to 7 days to cure, creating a water proof
If stone chips occur, and it penetrates into the bare metal only that chip area will rust preventing its from spreading if you were compared it to etch primer.
Polyester Body Fillers/bog/bondo and etch primer even Primer filler / Hifill / Primer surfacer all all porous.
Because of its slow curing, extra prep work required, its more ideal for restoration work.
For Restoration work we try to have the panel repaired as much as possible to minimize as much body filler as possible, Prepare the bare metal finishing it with 120 GRIT for a good bond, Epoxy primer the bare metal, once cured we give it a light sand and then we start on the Polyester Body filler work, because the Peroxide in the Hardener if applied straight
ontop of bare metal can corrode in the future, hence why you see rust under the bog when paint stripping panels. You can apply bog over bare metal but you will need to use metal conditioner prior to bog but Epoxy would the be ultimate protection right on top of the steel where it actually needs it !