A little trick I use to minimize the flash rust is, after you flush the tank with hot water and get it as dry as possible, spray the inside of the tank with WD40 and slosh it around. From a spray can it will make a fog inside the tank and get to all the nooks and crannies and also displace any moisture that is left in the tank. I have used this method on 2 tanks with good results but you have to be quick to spray the WD40 as soon as you think you have the water flushed. The only problem is you will have a film of WD40 so if you are going to use a sealer of some kind you will have to remove the WD40. I also have been using a product called Evaporust. It is non toxic and designed to remove rust in gas tanks and on tools etc.
Good luck
Sam