i know what you mean i used to have a black mgzt it hated tap water and i did the same used water butts. ive used auto glym for years their shampoo and they have a polish you spray on whilst the cars still wet very good. ive just subscribed to autocar magazine and they have a free gift of autoglym polish sponge and microfibre so thats saved me a bit.
I will have to try that
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.