bog will at least keep the rain out :-). Whether it is safe or legal I wouldn't say.

I'd be tempted to take the roof lining out, the roof rack off, wire brush what can be reached, treat the rust with ferrepro or your choice of rust converter, mate used to swear by a red water based latex paint after that, then I guess bog away, then prime and colour.

An alternative to rust converter etc might by a paint additive called Penetrol, which wicks in really well, and then sets hard.

I think the instructions for bog may call for it to be applied to bare bright steel - it has been a while.

There is a bog that has glassfibre in it, which helps in bridging holes.

You may find one of those cheesegrater body fill shapers makes things quicker - the trick is to start shaping as soon as the bog is hard enough, but not fully hardened. abrasive paper after that.