i would say it would have to do with how damp the filler got rust on the steel how bad? can you cut the rust with 120 grit in one pass by hand? or would it take 80 grit with a d,a if so at lest cut it back and refeather the filler. if bad you will see the rust work the feather end of the filler .if not it may be ok it is allways a good idle to put something on filler if it will set to seal it somewhat of air born junk oils and moister