That rust looks like it would come off with a toothbrush

Here is what I did for a tank with more rust that came out great......
Remove petcock, use a cork or some other method to plug the hole (I just used duct tape)
Get the works toilet bowl cleaner (or something similar, make sure it's acidic)
Have a box of baking soda ready
Throw a few nuts/washers in the tank, fill with toilet bowl cleaner
Carefully shake and slosh the tank for about a minute (you may not even have to go that long with your tank.
Drain, use the baking soda & water to neutralize the acid. Rinse extremely well.
Grab the wife's hairdryer and dry the tank (you can throw some denatured alcohol in there as well to aid the drying)
Pat yourself on the back for a job well done.....
I took a small amount of gas and 2-cycle oil and covered the tank with it, to avoid flash rust.