I noticed that my two gas caps are a bit corroded on top. The picture doesn't really show it very well as I zoomed in too much. But, on the top, there are various spots of blue/green coloring/corrosion. I was putting my cover on and didn't have time to try some general cleaning at the time. And now, I'm not going to be back on the boat for about 4-6 weeks. Can I clean/polish these myself to remove the blue/green corrosion, or do I need new caps?