I guess I just feel that driving rain, hail, wind and foot traffic are far more caustic than wiping the underside of a panel when cleaning. Both water and oil based polyurethane are going to break down too, and need to be reapplied as well. Go ahead and seal if it will make you feel better. Never hurts to have two layers of protection.
Also, how often do you have snake poo on the roof portion of the rack?
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