The gas tank of my first SRX600 was rusty. I eventually got all of the rust out using electrolysis and some phosphoric acid rinses. This took several weeks and lots of intervention. The electrode had to be moved, cleaned and periodically replaced. Also, I burned up a battery charger running it for days on end.
With this gas tank, I used rust911. When I started working on the tank, it only had a couple of pin hole leaks. As rust was removed, more pin holes opened up. I taped over these to keep the tank liquid tight. After a new pinhole opened up overnight and several gallons of rust911 leaked out, I stripped the outside and submerged the tank in a large bucket. The bucket was just big enough to immerse half the tank. I rinsed and flipped the tank every 24 hours. It took about a week to get all of the rust from the tank. I replaced the rust911 solution 6 or 8 times.
According to the instructions rust911 works best elevated above room temperature. I purchased an aquarium heater with a maximum setting of 93 degrees and cranked it all the way up to keep the bath warm. The higher temperature helps considerably.