Dennis,
Rust can be considered a growing organism. To kill it completely, all traces of it must be removed. This is why people actually cut chunks of sheet metal out...sort of like cancer. If your sanding method grinds off the rust on the peaks but leaves any behind in the valleys, it will most likely return. The choices are to blast it, which gets into the crevices, or grind off the peaks and chemically treat(neutralize) the inaccessible valleys.
Stuart
