What color are the stains? Are they rough or smooth? How high is the CYA and does the heater have a copper heat exchanger or is it a different material? Which Taylor kit do you have? (Not all Taylor kits are created equal). Did you add a metal sequestrant to the water after the ascorbic acid and have you continued to add maintenance doses of the sequestrant?
IF you provide this info we can perhaps better help you.