Traffic film is not only dust; it holds oils and salt residues. Alkaline pre-wash breaks this film, but it also frees chloride ions (salt). If the solution dries, salts stay concentrated against aluminum and steel. That speeds hidden corrosion on brake lines and heat exchangers. Rinse thoroughly before contact wash, especially after winter road treatments. Use cool panels and shade to avoid fast drying and streaking. Foam thickness is not performance; dwell time and coverage matter more. Soft water improves action by keeping surfactants (cleaning agents) active and mobile. Check rinse water sheeting; slow sheeting suggests remaining film there. For controlled dwell and safe removal, see Car & truck cleaning concentrate.
