First, REMOVE YOUR PETCOCK. Then get some hose and rig it up like this. break>
After that, degrease the tank with dish soap (like Dawn) or another degreaser of your choice. If the tank is removable, then I'd throw something in there and shake it up to help get the big chunks off. If it isn't removable, I'd go for a high pressure hose at the car wash.
Then add the works. Do not get any of the acid on paint. Or chrome. Or plastic (like the measuring cup on the gas cap -- I used a plastic bucket to hold the acid, and that was fine though). Or concrete. Do not get it anywhere or on anything that is not the inside of your tank. I think I used one bottle and filled the rest up with water, but you can use two bottles too if you want. It will work faster, but it will also destroy other surfaces faster if you happen to get it somewhere it isn't supposed to be.
After all of that, rinse out the tank well. Then use baking soda to neutralize any acid left. Then rinse the tank some more. Then rinse the tank out out with rubbing alcohol to help get rid of the extra water. Then dry it out as best you can. Then IMMEDIATELY either fill it up with premix, or hose it down with WD40 to keep it from re-rusting.