Soap scum and streaking are caused by flash evaporation of the surfactants and polymers within a car wash soap. When a vehicle is washed in direct sunlight or on a hot panel, the water carrier evaporates faster than the cleaning agents can be rinsed away. This leads to surfactant crystallization, where the soap solids bond to the clear coat or glass, creating a hazy, white, or iridescent film. Unlike mineral water spots, soap scum is a chemical residue that requires a high-lubricity emulsifier to break down the dried bonds without scratching the paint.