“…..rinse with hard water actually helps drying much better than spot free. Hard water increases the surface tension of waTER making it easier for it to bead while spot free reduces the surface tension of water which tends to make water puddle. Years ago operators used to apply a fine mist of tap water and call it a setting rinse.”
Puddles occur because a liquid with a high surface tension (hard water) pulls more strongly on the surrounding liquid than one with a low surface tension (soft water).
The phenomenon is seen with tears of wine on glass.
Also those carwash operators that used setting rinse years ago mostly wiped up excess water with towels. Otherwise, hard water tends to leave spots.
For example, dishwasher rinse aid contains special surfactants that prevent drops from forming so that water drains from the surfaces in thin sheets rather than forming droplets.
Mr. Clean Auto Dry Car Wash system uses such special surfactants. Wash car, rinse and let air dry. No wiping.
Today, most car washes use forced air to dry cars, no towel drying and no labor.
Here, drying agent or wax (natural or synthetic) is used to create a high surface contact angle that prevents the water from sheeting – spreading out over the surface and harder to dry.
“my top wash” presoak 1, presoak 2, hp rinse, lo ph foam polish, hp rinse, medium pressure teflon sealer wax, spot free rinse and blowdry.
This could be pre-soak (high pH), foam polish (low pH), high pressure wash, low pressure wax (DB, Rain-X, etc. are typically low pH), spot free rinse and air dry.