Surface rust will not bother the POR 15. They have a special cleaner, but I used phosporic acid etch (available fron auto paint stores) as I already had some. Braze is a good way to fill gaps between sheets if the sheets lap... this is done at the factory ... well at Kawi for sure. I have also seen Yami & Suz fuel tap bolsters brazed in.... good stuff. Good luck.... looks like a great project.