I do not know how serious or obsessive you are about this issue BUT If you go to griotsgarage.com you can buy a portable water deionizer for your hose. They work super.

Don't know how handy you are or if you own a house but if you do consider installing a water softener and run one of the hose bids through it and that is where you can wash the cars. You will be amazed at how the shower feels if you have a water softner. I plumbed the garage for hot and cold water to rise the snow and salt off the cars in the winter.

Other suggestions may be cheaper. Just some thoughts or for future reference if someone is looking at a new build or major remodel to a man cave garage.