Anyone have tips on what product I could use, besides painting, to prevent corrosion between multiple layers of metal? I was thinking of applying silicone between each sheet and cleaning up where plug welds would be present and applying silicone again after the welds cool.

 

corrosion-X works great

 

Weld-through primer
JB Weld
Panel adhesive
Solder
Brazing

 

On military aircraft, we used mil-S-8802 sealant, which is essentially just a much more foul-smelling version of silicone sealant mixed in a two part compound...it was spread in a thin sheet between the metal sheets (primarily aluminum) to give rigidity, corrosion control, and resistance to the flexing that pressurization causes. I'm sure you can find something similar, or simply prime the interior surfaces with a zinc chromate primer before joining the parts.

On aluminum we used alodine as a chemical conversion coating, but we didn't weld after, as we primarily used solid rivets. I'm sure there's probably a chemical conversion coating for steel, but I'm not familiar with what specifically it is...