It will burn off around the weld. If the tubes are sealed (no holes in them like a jeep frame) it wont rust from the inside out anyways. And even if you did por15 the inside of the frame the metal has to be clean with no oil on it and it would be pretty hard to clean it all out or it wont stick well. It would be easier, cheaper and more than likely better thing to do would be to oil the inside of the frame with bar and chain oil.
Eastwood makes an internal frame coating that would be better than POR 15. POR 15 is great at coating rusty metal, hence the "Paint Over Rust" part of the name, but it isn't that good at coating clean new metal. Their activator actually uses phosphoric acid to flash rust the metal so it has something to stick to.
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