For anybody that needs to clean out a gas tank, here is a tip that might help.
I just got done cleaning the rust from the tank of my M715. It was pretty bad. Lots of goo and very thick rust. I cleaned the goo with lacquer thinner which I sloshed around and soaked overnight.
For the rust, I used electrolysis to remove the rust. (I learned about on this BB) If you are not familiar with the process, it simply uses a solution of Arm and Hammer Laundry Soda and water which is energized with DC voltage (car battery charger) This process chemically breaks the bonds of the rust to the steel without damaging the base metal. The advantage of this process is that it removes the rust everywhere the solution touches. Because water fills every available nook-and-cranny, it removes rust in hard to get to places, (like the inside of a gas tank).
I constructed an electrode inside the gas tank out of steel rebar suspended from the tank with closed cell foam. Filled the tank with the solution and connected it to DC voltage from my welder and let it boil for a few hours. When I was done, the tank was completely free of rust.
Total cost for the project was about $10
Follow this link for directions on electrolysis:
http://antique-engines.com/electrol.asp
http://myweb.tiscali.co.uk/andyspatch/rust.htm
http://antique-engines.com/electrol-details.asp