Backside of patchpanel?

I am in the process of welding patchpanels on the bedside of my truck and was wondering what is the best thing to put on the backside of them.It's the S10 so yeah,,,:CRY: the entire length needs replacing.So,,, I can get to the back of everything (after welding)except the wheel arch,,,it fits up against the inner arch.So I can't treat/paint the wheelarch after welding.
I have never used weld-thru primers because I have always doubted their effectivness.Do any of them really work? Or is there any product on the market that will with stand welding and keep rust from developing?

Everybody has their opinion as to what is best. I would paint the backside with POR15 after welding and maybe spray POR15 in the hard to get spots. Spray might be the wrong word. I have in mind to make a wand out of flexible copper tubeing with the end pinched almost shut or with a nozzle on the end. Use low low air pressure to blow POR15 in the hard to get at seams.

Haven't tried that yet, so it's only a theory.

Weld on primer isn't that great IMO and don't trust it either. POR15 will work. POR15 it first and leave the front part bare metal so you can still weld it. Put a strip on the bed and only weld on half of it and put the new panel on the other half... Get what I mean? The tab or strip will be BEHIND both pieces you are welding together.