These rust stains are rust film. It does not damage the material and can be easily wiped off with a sharp kitchen sponge and "Oxi Action" or Steel cleaner. If this happens to you, don't wait too long before cleaning to avoid damaging other plastic parts.

OK, but how and why does this rust film form at this particular point?

Depending on the alloy, metals react more or less strongly with salt water - this cannot be avoided. We use stainless steel (V4A) and titanium to prevent rust formation as far as possible.

We recommend rinsing the bar with fresh water after the session, drying it completely (the bag must of course also be dry) and storing it in a warm and dry place. Damp cellars are not suitable for long-term storage of kite material.