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Jim, if the stanchions are not stainless steel, but something else that has been plated, what you are seeing is a failure of the plating. The actual metal stanchion may be plated with two or more different materials before the top layer(s) of chrome, and any of those layers can fail from improper application or contamination, or simply aging.
You can scrape/chip/sand at the spots and then either paint over them over or put some stainless steel tape (sold in auto stores to cover rust holes in trim) over the spots to hide them, but the permanent solution is to remove the stancions, take them to a local chroming/plating shop, and have them stripped, polished, and replated. Of course that may make the others look old--so you may want to wait for the off season and pull them all.
If you tell the plating shop "I don't need these for a couple of months, you can run 'em with the next batch you do whenever that is" they may also cut the price for you.
If you really want to get ambitious, there are some DIY plating solutions sold on the web, that might be a cheaper way to deal with the spots, or just the spotty stanchions.