I've had to use the Kreem kit, 2-3 years ago.
So far, no problems.
USUALLY, rust forms in the tank from condensation, due to the bike sitting for a while with less than a FULL tank of gas.
There was one guy who had water in his tank from the gas tank fill area drain line being plugged. When it rained, the water accumulated around the tank filler indent, and seeped into the tank.
If the tank isn't LEAKING gas, I wouldn't get overly excited, but would go ahead and use a KREEM kit, or one of the other gas tank sealers.
You'll either solve the problem, or buy a lot of time, while you search for a used tank, of the right color, or ebay.
Do you actually have exterior rust on the tank? I kind of doubt it's related to the internal rust. (Otherwise you'd have gas allover the place...)
The KREEM process will take you at least a long weekend, and is a little messy, but it's not difficult.
I think there's still a product report I wrote on using the stuff....but you'll end up using a bunch of pennies, or washers, nuts, or something, inside the tank, with some soapy water, to knock the rusty stuff off.
You'll seal the tank up, and pretend it's a big cocktail shaker.
http://forums.banditalley.net/viewtopic.php?t=338 There's a least one more product, from Yamaha, that some people like.
Some people have claimed the KREEM stuff flakes off after a couple of years, but I suspect they rushed the process.
Mine's been in there for 3 years or more. I did stretch the process out to a full week, giving the stuff plenty of time to dry.