How best to defeat old hard water stains on my car

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have water spot remover from Megeirs, but to no avail

i'm trying to find out if my carwash area is hard water or not, my mom's Lex was spotless, did dad's SUV to no avail and there are scaly old spots which the paid carwash he takes it to does not remove.

next up is vinegar and water 1:1, but man, these scales are tough.

adding some baking soda to the vin/water is suggested by google, so there's another option.

Anyone have good experiences with products or better, home remedies?

Mahalo!