Hi. I've designed and built a unique treehouse/swingset/climbing structure for my children. It has a couple of aluminum components,such as a "high bar" made of aluminum pipe. The problem is, when the kids hang on the bar, the aluminum makes their hands black. I've tried rubbing it down with fine steel wool, cleaning it with alcohol, and rubbing it with an automotive polishing compound. Only helped a little. Is there a way I can treat the aluminum to solve this problem - either a different way of cleaning it or some sort of coating I can apply? It would have to stand up to the elements and not flake or chip off. I think an ordinary paint would likely flake off at some point, possibly creating more of a hazard to the kids than the aluminum rubbing off on their hands. Any suggestions? Thanks!