The manufacturer says it lasts a year on the underside of a car. They also say that is does not wash off that easily - not washed off by 35 psi water -.

What is your experience?
How do you tell whether it is washed off?

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.