I found a bottle of 'MetalPrep' at a specialized paint store for $10. You mix
with water 50/50 and put in parts for a few minutes - goes into the pores and
preps surface for better primer adhesion. Still doesn't get all the rust out
- but more penetrating than sand I guess. On some stubborn rust I poured it
on full strength and scrubbed at it. For other stuff I've used rust converter
products like 'Extend' before priming - hopefully in 10-20 years I won't have
too many rust problems popping up...but not counting on it.