There are different grades of stainless also and the lesser grades will rust too. Even high grades that are contaminated with carbon steel will get rust spots on them. Contamination can happen simply by using the same saw or file that has been used on carbon steel.
My roof rack and fillet table bars have rusted even though I used 316L grade tubing. I have wondered if there is anything I can do to prevent this. Is there a conditioner oil or wax I could apply that would make a difference? I notice the bow rail never looks rusted but it also kind of looks like it’s been electro polished.