Sorry for the poor analogy...but its true...:
Rust is like a cancer, if it cant be sanded off, it must be cut out..
Painting any magic stuff on is like putting a plaster on a sick person..
If you get it sorted properly now it'll be a job well done...
If you bodge it now it'll come back to bite you in the arse...
Thats just my humble opinion of course, after over 30 yrs body repairing cars....