[Flux core... should I just give up now??]

A copper backing plate helps A LOT. But I think you'll still need 023 wire w/ gas.

[filler before or after rustseal/por-15?]

I recently applied Bondo over POR15 and it seems to adhere just fine. Just scuff up the POR15 first.

 

The problem is that when you sand the Bondo you'll end up sanding through some of the POR15 too. So you'll still end up with some bare patches.

[3M's Weldable Primer - A+ Product!]

I recently tried the SEM Brushable Weld-Thru Primer, and I didn't like it. I found it very hard to strike an arc, and there was too much spatter. In the end I gave up on it and used a butt joint instead.

 

If you want it locally, try an auto body paint shop.