Done a lot of tanks this way. Always used a plated bolt, and that cleans the rust out. But also adds a coating to the tank while doing it. Did a Voyager 1200 tank winter of 2019. Turned out like new, and did not harm the factory paint on the tank.
I only used water and Borax with my battery charger. In 12 hours the bolt was almost gone. But the tank was spotless and it had sugar rust coming off when I started.