In the collision industry filler over bare metal is the standard, even thou some car makers say to retain factory warranty epoxy is applied first. Filler on bare metal has been the standard for around 30 years.
In addition, most filler jobs are never exposed longer then a day or two without being primed or painted. I can have a quarter panel cut off and a new one installed sitting in paint before the end of the day.
The home user, I highly doubt can finish a panel and prime in a day or two. Thats why you see epoxy been used first, its mainly because you are preventing flash rust from developing because you have metal exposed for long periods of time.
It has little to do with the filler absorbing the moisture but having bare metal exposed and starting to rust and you dont even see it.
In addition, filler sticks better to fresh epoxy then metal.
The half that says filler over bare metal are probably collision guys, the other half that goes with epoxy are probably resto guys. Sometimes what the collision industry uses and does is NOT the best route for a restoration. Collision is all about speed and often quality and durablity suffers, you want products and techniques geared towards that mentality?...Eric...Collision technicain.
P.S. all my work at home gets epoxy first then filler.
