Most of the time it is not visible: if you don’t see them, take a common plastic shopping bag, the disposable ones so to speak, and pass it, with one hand inside, over the bodywork: you will feel the bodywork underneath rough. What you feel are the contaminations. On the bodywork, if the contamination is deep, it can even be visible: the color of the bodywork will appear dull and in the case of white cars you will still see brown dots after washing.
