I use pool acid [muritic acid] in a plastic cup/bowl in a Well Ventilated area [Outside] & wear protective Glasses & Gloves. Sometimes have to leave overnight, pour off acid then use Baking Soda & water mix to Neutralize the acid. Quickly dry w/ a torch or heat gun.
Also works great to remove concrete from bases.
NEVER use on Alum, Pot Metal or Galvanized Metal