I've seen a few different methods used for what product to use first on bare metal surfaces so I'm hoping I might get some insight as to what is best and why. I'm doing an all over but the rear panel was replaced and the two vertical seams were welded together, ground smooth and sanded with 180. Should I epoxy first then apply filler? or fill over bare metal then epoxy??.... I had planned to shoot the entire car in epoxy primer first then, filler over the mentioned areas, then high-build primer, block and topcoat.

either way works. the best in my opinion is to epoxy then do your filler work. then you can spot the filler area with epoxy and block, then shoot the complete with HI build. be sure you read the tech sheet for the epoxy about how long you have to put the hi build on. there is a window of time when you don't need to sand it, most are several days like 3-4.
Jay D.

^ So you are saying to epoxy over everything, do filler work then re-epoxy over the filler work, then high build primer and block, topcoat....? I guess that makes more sense. I didnt think about re-epoxying over the filler.

you can ^ or
just spot the areas that need body work with epoxy, do your filler then coat the complete with epoxy then hi build. i'd probably go with this, it's probably less time restrictive than above. it just depends on how long you want your bare metal exposed.
or
do the complete with epoxy do your filler work then shoot 2-3 coats of hi build.
there are several other ways but these are probably the best.
Jay D.