Originally posted by Centerline
I disagree. Using body filler over bare metal promotes rust. The filler is porous and it can and will trap moisture under the filler which will allow the surface to rust over time. This is the exact reason for using either a self etching primer or epoxy primer over the bare surface prior to doing bodywork.
Your right the best idea is always put body filler over epoxy but in his case (learning it really don't matter. Long term 10-15 years it will. Most body shops still put filler over the bare metal for everyday production work. I always spray epoxy first but only thing I do anymore is show or restoration.
But you need to educate yourself on the acid etch primer as far as putting body fuller over it. Do it yourself but don't give that advice to other people as that is a major problem waiting to happen both short term and long term.