There is no paint or rust proofing product that will hold up to the heat from welding. I think you are going to want clean metal to weld or any impurities will affect your welds.
I like the POR products, but you might get other recomendations. they have worked ok for me. just have your area clean and degreased, I use their Marine Clean. their Metal ready etches the metal for good paint adhesion. give it a couple coats and maybe use their seam sealer. I have used an acid brush to work it in.
For an under coat I bought some of that bed liner brush on stuff, dries to a tough surface. don`t remember the name of the product, but worked good.

Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works