I used a product in a friends MG tank that worked great.  It does not hurt paint and does not etch the metal - it only loosens and removes rust.  It is non caustic and you can dip your fingers in it......doesn't hurt a thing.  Evidently the way it works is it removes the oxygen from the rust.....and the rust just falls off.  I left it in the MG tanks for about a week - as the tank was badly rusted.  It came out squeaky clean - then I used POR 15 fuel tank coating to prevent further rust caused by the wonderful new ethanol fuel.

Here is a ling to some products that are the same:

http://www.rustremover.biz/

http://www.evaporust.com/evaporust.html

http://www.theruststore.com/Evapo-Rust-Gallon-P1C1.aspx

When you rinse the tank out - use hot water and then a blow drier or air compressor to dry the tank as quickly as possible.

I used oxalic acid.  Worked pretty good.  The tank wasn't severely rusted, but still too nasty to use.  A couple go rounds with the acid and it got rid of the rust.
Finding it locally can be a pain, but I got it online at flea bay

Ive never even heard of it, but it looks promising.

Hey! Seems I went out with a girl like that once..

& now I cant remember if I was disappointed,,
Ohh well,, back to taking rust off,,
Looks like dumping some in & sloshing it, letting it sit a while, & doing it again if needed wouldnt be too tough, maybe layin the tank down on one side & let it soak,, then the other,,unless its cheap enough to fill the tank?
Ill bet its not cheap, tho,,


Hmmm,, 35 bux a gallon ,, online,, & a quart of Ospho was about 12 last time I needed some,, & wouldnt need more than a cup to do the tank, unless its not just a little rusted,

Just how bad Is that tank? Is it pitted & has scale inside? If it was something you could lay hands on, could you wire brush it off? I mean by hand, not a wire wheel on a drill,,,

If the inside of the tank is just lightly hazed over, ignore it. Add an inline fuel filter.

If it is crusty, evapo-rust or similar. Fill it to the top and cap with a plumber's test plug so you can turn the tank upside down or hang it by the rear mounts.

Look like somebody filled it with soggy Copenhagen? Fill halfway with kerosene, throw in some 3/8- crushed rock or a handful of brass nuts and shake it like you want to pop it. Then hit it with evaporust or such.

Easiest way? Find a real radiator shop to hot tank boil it out. They can solder any pinholes that crop up. And add a real lining (unlike that Kreem crap). Went this route on Lisa's 1971 CL100 tank. It was either that or throw the bike away, some parts just are not available anymore.

lite rust on the fill neck - heavier rust scale at the 1/3 full line  - no wet nasty copenhagen looking stuff as far as I can see from the limited view from the fill neck - bike has a fuel filter on it - bought a new one to repolace the original one after i de-rustify the tank

i am leaning heavily towards the evapo-rust - just wish a few have used it would hollar.....next rainy no riding day the tank will get soaked inside with that stuff - provided i dont hear some horror story about ti before then.

How much scale is in there?