Using WD-40 to Clean Tub Drain

The tub drain had hard water marks that I tried different cleaners on and they couldn't do the job. A friend suggested spraying WD-40 on it and by using an old toothbrush the hard water ring was removed with some scrubbing. The corrosion mark on the chrome drain was caused by a harsh cleaner quite a while ago and I sure wish that WD-40 was tried first long ago and then that damage would have been avoided. Now I know try WD-40 first.