This is probably not a "purist" response, but there are also several "rust-kill" primers that supposedly turn remnants of rust inert and seal them so they're "deactivated." I know a couple of highly-regarded auto restorers who swear by these products, and I've used them on a couple of old bike frames myself. There are times when you just can't get to the stuff in the nooks and crannies, and you have to reconcile yourself to a reasonable compromise.