I've helped clean and seal alot of old Harley tanks for some buddies. It's fairly simple, just time consuming. Just need some time, beer, and buddies that's willing to shake your tank around for a couple of hours.
![Yes Nod :yesnod:](https://board.marlincrawler.com/Smileys/marlin/yesnod.gif)
But the key trick is like Bleak said, you need to be able to knock off all the rust off, or you won't get a good coating.