i repaired my roof, had 3 tiny spots behind the sunroof. ground the rust out and plug welded them up.then repaired and painted.
stripped the whole sunroof out. removed all traces of rust with wire brush heads etc then treated it withe P O R 15 acid rust converter/ treatment. then P O R 15 paint.
all the above items are available from frost.
the paint is brilliant, it actually gets harder in damp conditions. its used on boats etc.
it was a shitty job, but beats replacing the whole roof

that was two years ago, so far so good.

hope this helps