A large portion of the discoloration I see has got nothing to do with the metal, which is normally a genuinely latent aluminum shrouded in aluminum oxide. What seems to happen a considerable amount is flying creatures sit on the aerials and simply do what falls into place without a hitch - abandoning somewhat of a stain. I put in a couple of years adrift and saw a gooney bird perch on a pivoting radar elevated medium-term. The following morning the deck underneath was a wreck of semi-processed fish, an enormous sum for only one winged creature!