by devil » Fri Jan 19, 2007 10:50 am
A very high pressure "touchless" wash will cause more damage than a rotating sponge brush. If it doesn't, it means the pressure isn't high enough to effectively clean the car. Technical reason: crud (as opposed to loose dust) forms a hydrogen bond to the paintwork. To remove the crud, you have to apply sufficient chemical, physical and mechanical energy to break that bond. Mechanical energy (e.g., the rotating brush or sponge) is the easiest way. If you don't have that, you have to rely on the chemical and physical energy. The chemical energy is supplied by the detergent which has two actions: a) it forms a micellar bond to the crud and removes it like an onion skin, layer by layer, but the micellar bond is weaker than the hydrogen bond and it leaves an invisible single layer of crud molecules over the cruddy area, and this will be a primer for new crud to develop in the days that follow and b) the detergent itself, if strong, will form an invisible monolayer which will enhance subsequent wetting of the car by dirty spray from wet roads. The physical energy is a matter of surface tension differences between the detergenty water (typically about 25 dyne-cm) and the rinse water (pure water is ~72 dyne-cm), combined with the kinetic energy of the water velocity, which is relatively small and mostly dissipated by rebounding. IOW, rotary brush carwashes are more effective at the removal of crud. The best compromise is the manual wash by skilled operators with good sponges and leathers.