I have a 16 year old boat in relatively good shape with the exception being the toerail. It is black annodized aluminum and is pretty well beat up. The finish is gone in some areas and peeling in others, while some areas are still fine. The finish now is a dull black. I guess the only option is to sand & paint it. How much of the current finish needs to be removed? Do I need to go all the way down to bare aluminum? Can I just sand the splotches and paint the entire rail with polyurethane? Will the poly adhere to the annodizing?