Same as any other rust. Remove all the rust with a grinder, wire brush, sandpaper, etc. Weld up any major pinholes and grind flat. Then its all refinishing work. More than likely finding a replacement is the best method. The sunroof rust from the inside out, so there is most likely a lot of rust towards the bottom. Its not too uncommon to see rust all the way around the sunroof that hasn't popped to the surface yet.