Thanks for the good advice. Actually I live in Oregon and have a 88 Plymouth minivan that I bought new when I lived in CO and the underside looks better than most 5 year old MI cars so no, I'm not too worried about my 54 rusting out.

With all the advice here this is what I'll probably do. 1) spray some weld thru primer int he crack between the to layers, 2) clamp the backer plate down tight and tack weld the edge, maybe 1/8" welds at about 1 inch O.C. 3) sand blast the whole fender, 4) epoxy primer the whole thing, 5) rubberized undercoat the fender and running board (top side and edges of the running board get bed liner), 6) paint the fender, 7) bolt it together maybe with a thin rubber washer between the running board and fender, 8) add a brushed in coat or two of rubberized undercoating to the backer plate area.

If I use seam sealer should it go on before or after the epoxy primer?

 

Chassis Paints, Rust Paints & Chemical Resistant Floor Coatings use it as a weld-thru primer/sealer/seam sealer and paintable and bodywork can be done within any stages of using this stuff. (master series coatings) silver primer/sealer.

 

They make seam sealers for primer and bare metal surfaces. I have both. Go to 3M's website and take a look around, that was what I did, then bought what I needed at Summit Racing.