Tank cleaning threads are ALMOST as volatile as favorite oil threads...
My current favorite is the vinegar route. I cleaned a severely rusted, severely varnished CB200 tank with it and it cleaned it right down to beautiful gray steel.
I removed the zinc / aluminimum petcock because the mild acid will eventually damage it. I made a patch for the petcock opening with plain flat bar stock and a homemade gasket. I filled the tank with straight, pure white distilled vinegar, right from the grocery store at $2 a gallon. It sat in the tank for 3 or 4 days and was already so contaminated that I dumped it out and did a repeat treatment for another 3 ~ 4 days and it absolutely cleaned the tank a treat.
When you pour it out, however, that raw steel will flash rust right before your eyes, so I rinsed it with about a quart of gasoline swished all around and dumped out. I got only a slight bit of 'yellowing' but not a spec more. I've not had a single problem in the several months / 500 miles since.
In my case, one nice thing is that this treatment will not damage the paint, which was important to me as this was an excellent-condition, original paint tank that I wanted to preserve. Just terrible internal rust, and at least 1/4" thick varnish goo in the bottom.
Here's a link to the thread where I documented this over on the HondaTwins forum.
http://www.hondatwins.net/forum/viewtopic.php?f=19&t=1376&st=0&sk=t&sd=aGood luck with it, however you choose to go.
Kirk
