You can paint the exterior, but if the joint is at the pillar the interior cannot paint, and rust can come from there.
I did try to save the roof. When I saw small bubbles came out, I had them rectified. My spray painter grind it down to bare metal from the top, paint anti rust, putty, primer and paint. For the interior he remove the roof cloth and scrap out the gum, grind out to bare metal. paint anti rust paint, gum back with different gum, and paint and hook up back the roof cloth lining. It worked for 6 months or so only, then the small bubbles were back. I Intended to keep but since the bubbles came back, sold off.......lol