Steelflex isn't going to keep a perfect seal if the rivets are really loose. I'd have to bet they'd eventually vibrate/break free and start allowing water in again. If the rivets aren't in too bad of shape, you can get away with it, but from my perspective it seems like you should rebuck the rivets, or seal them from the inside with some epoxy or something. However, this is just my opinion. People have had luck with gluvit and steelflex, but I'd rather fix the problem than throw 150 bucks worth of specialty paint at it.
With the decks in, your only option is to visually check the rivets from the outside. If it's apparent they need serious work, you're probably going to have to pull the decks out.