For some reason I've found the need to wash the M3 far more often than any other car I've owned. Being black means it looks great when clean, but pretty grim when dirty. Giving the car a foam and pressure wash is quick and easy, it's preventing water spots that takes the time. I've got a big bundle of microfibre cloths (good value from Screwfix - currently £25 for pack of 50:
https://www.screwfix.com/p/microfibre-cloth-blue-50-pack/62624 - no connection, other than as a happy customer) and they are very good for drying the car off, but it's still a bit of work.
I've found myself looking into getting a DI unit, just to rinse the car off. I made up a car wash trolley a few years ago, as my ancient Clarke power washer doesn't have an wheels or stowage for the cable, hose, etc, and there's room on it for a small DI vessel. The plan is to plumb it in so that there is a bypass around the DI vessel for foaming and pressure washing, that can be turned off to run deionised water through a low pressure hose for a trickle rinse.
I'm hoping that a gentle DI rinse will allow me to give the car a quick wash, without the need to dry it, in order to prevent water spots. Our water here is fairly hard, and the outside tap is fed directly from our borehole, before the water is treated and softened, so has a total dissolved solids (TDS) reading of around 235ppm.
The DI vessel and resin is a bit of an investment, roughly £50 for the vessel and fittings, plus another £75 for enough resin for two refills, but that set up should give over 2,000 litres of deionised water, so enough for quite a few car rinses, plus it may be handy for cleaning the house windows (I've been thinking of getting a water fed pole for a while for that).
Has anyone here experience of using deionised water for spot-free rinsing?