Rust between welded panels

I there some magic goop I can spread on the hidden surfaces that will prevent or slow rust after the welding is done?

Thanks,
Tom

You will need to grind away the rust 1" on either side of the weld "before welding", then apply cold galv after welding.

What can happen, more often than not, when you weld over rust and scale it leaves been those impurities with-in the weld thus leaving a very weak weld joint. When you weld over oil and grease that adds carbon to the weld thus causes the weld to crack when cooled or the weld becomes very brittle.

T_Bone

I really doubt I can weld it air tight, so I was worried about it rusting from the inside out. Do you know of any magic goop I can apply to these soon-to-be-inaccessible areas BEFORE welding that will tolerate the heat, and keep rust from forming?

Thanks,
Tom