When using the Kreem kit, available at most shops for around $30, you do have to remove your petcock and seal the hole as well as not have your gas cap in when doing the manual agitation.
Be sure and rinse well imediately as it can do a real number on 30 year old metalic paint. I'm assuming this is about the bichen Honda mini.
There is a draw back to Kream though in that eventually you might have to go through the process again. If you can find a radiator shop that recoates rads to stop leaks, the same interior coating will work on gas tanks. Lot's of Vintage guys consider this akin to blasphemy, but it costs about $25, is a one time application, and eliminates any pin hole leaks that may be present.
There is a shop called Elizabeth Radiator that does this. They had a web site but I keep getting a "domain you are using is not configured" or some such message. You could do a google search and give it a shot.
Oh yea, been meaning to send this link to you since I saw the post and pic. These guys have lot's of stuff for you. Heck, any mini fan ought to check these guys out........
>HERE<