Anybody hear from Don?
He was telling me last summer he had an answer for this problem!
NOW, if you want the real answer, once and for all, streaks, etc. will wash right off. Super Clean your trailer, have your local body shop shoot two good coats of a 2-part clear on it.
Benefits are numerous, will even stop steel rivets and hinges from rusting. Finish will last for years and either the black streaks will disappear totally, or they will wash right off.
DV...Where in the world is Don!