Glass bead blasting is very gentle to metallic services. You could also try soda blasting. You can buy small hand held blasters that run off small air compressors. Google it. Otherwise take them to your local shop and have them send them out for cleaning. It looks to me that the stickers that use to be on there caused the build up by trapping water behind them. A little Vinegar or CLR might actually do a pretty good job.