You got a bum job. Radiator shops used to use some very caustic cleaning solvents in heated "boiling" tanks to dissolve the deposits in copper-based radiators. They didn't actually boil the stuff- - - -it was heated to 190 degrees or so. Those chemicals got scarce, expensive, and tightly controlled by the tree huggers and other environmental whack jobs some time ago, so getting a radiator or gas tank chemically cleaned is just about a thing of the past. Aluminum radiators would just sort of disappear in a caustic tank, so most shops have gone to some sort of detergent-based cleaning solution these days. That stuff is pretty much useless for getting rust out of a gas tank. Use a strong mixture of Sani-Flush toilet bowl cleaner and very hot water to do your own cleaning, then flush it with several rinses of water. You'll probably do a better cleaning job than the radiator shop- - - - -just don't tell anybody where you dump the stuff!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!