You can find all kinds of comments, products, procedures on the web to clean a rusty tank which will all drive you crazy after a while. I decided that I want to deal with something that is non toxic since I am kind of a slob and dont want to ruin any paint on the tank or wear protective gloves, etc. I bought a gallon of EVAPO RUST at pep boys for 25.00 and it worked great. It worked so well that I dont even need to seal the tank after soaking with the EVAPO RUST. I just plan on keeping the tank full of gas so rust does not have a chance to start. Before you use the EVAPO RUST you have to prep the tank. Mine was a Sport 90. Remove the tank from the bike and take off the petcock. My fuel line which goes under the tank and connects to both sides was so rusty they were clogged. I used some gunk engine cleaner sprayed on a large heavy duty safety pin and picked away through the little nozzles until they could go through into the tank. That took a while but you want to get all that crap out so you can replace the fuel line that connects to both sides when you are finished. Now for the tank. I used HOT WATER and DAWN detergent along with 15 large bolts and 5 large nuts put into the tank. Take an inventory of the number of bolts and nuts you put in the tank and write it down. Trust me on this! You want to make sure they all come out because they could wedge them selves periodically. I put the gas cap on the tank and my hand over the petcock hole and shook the bolts around to loosen up all the caked on rust. Empty the water out into a large clear or white plastic bin so you can monitor what comes out. Keep on filling, shaking, rinsing, draining, etc, until you can hardly get any more flakes or debris. This takes a long time, It took me about 20 fill ups and three hours of work until I was satisfied to go to the EVAPO RUST. You should have little or no flakes (only a slight amount of small specks or debris) before starting with the EVAPO RUST. Plug up the petcock hole. I wrapped plumbers thread tape around the threads of the petcock hole to protect the threads and then heavy duty packaging tape around the whole petcock hole and then a small hose clamp around the tape to close it tightly. It leaked a very small amount but that is ok since you will be placing the tank on top of the plastic bin. Place the fuel line back onto the each side of the small nozzles of the tank so nothing leaks now on the bottom of the tank. Place the tank on top of the plastic bin and dump the whole gallon of EVAPO RUST into the tank. Cap the tank and allow to sit overnight. Next day turn the tank over and allow to sit overnight. Next day turn tank on one side and allow to sit overnight, Turn tank on other side and allow to sit over night. The time all depends on how rusty your tank is. I let it sit at the bottom another night and on the top another night so my total soak time was 6 nights! When you are satisfied that it is clean, drain the EVAPO RUST through a fine screened filter so you can save it and put in back into the original container. Rinse the tank out with water using your highest pressure stream from your garden hose. I did this a couple of times and shook all water out. Then cover up the petcock hole and bottom hose again and put the EVAPO RUST back into the tank and shake it all around. You do this to prevent flash rusting of the interior surfaces of the tank. Drain the EVAPO RUST back into the original container. Now you need to just air dry. I used my air compressor and my wifes hair dryer (dont tell her!), or you could leave a fan blowing on the tank. Make sure all holes are open and the tank is completely dry before re installing. You can see the before and after pictures. Amazing! Total cost was 25 bucks and you can use the EVAPO RUST to clean other parts. No need for harsh chemicals, tank sealers, multi-step processes, etc...Good Luck!