Originally posted by troy-curt
Anything you apply to bare metal to seal it will trap moisture, if it is present at the time of application.
Of all the thousands of gallons of filler that I have used, I have never had any problems with moisture or rust underneath it when applied over bare metal.
If you have moisture problems, you need to use some kind of humidity control.
I agree, however a lot of shade-trees wind up doing bodywork over several weeks (or even months) and the surface never gets protected until the car is totally ready for its first overall coat of primer. These are the people that need to be concerned about body filler over bare metal.