So here are the main points I remember from the lost thread. Distilled water- No minerals. Deionized water- Could have some minerals. RO water- Will have low mineral content. Tap water- Mineral content depends on how "Hard" local water is. The main reasons one would use anything other than tap water is because local tap water is hard, no shade to wash car, no Hydro or other spray-on-wet wax is used, don't like drying, or some combination of these. I had started the other thread to see if anyone was using distilled water to do a FINAL rinse, spray on Hydro, then let it all air dry. I tried doing it that way, and the Hydro looked like crap. Our local water is not very hard, and because I use Hydro with every wash, drying is a must. If I blow water out of the trouble areas using compressed air, I end up with little to no water spots/dribbles. If there are any spots, QD gets rid of them. One of our club members here, who is a clear bra installer, has an RO system in his shop. I have used it, and it works great if you just let the car air dry. He does not like drying. Mark