gasoline is dangerous.. one spark and you're a candidate for a Darwin Award.. Anyway, wait for the kit.. it'll tell you how to do it.
After cleaning with their "Marine clean" or whatever to remove old fuel varnish and after the acid wash to get rid of residual rust and acidify the metal, i think they tell you to water rinse thoroughly and a complete dry with warm moving air, like from a hairdryer, before applying the tank sealer..
So, since you're gonna use water at some point anyway, knocking flaky rust off with soap and water sounds like a good plan. I've done it when working on my tank and soap/water is safe and free and unlimited in supply.