Rust marks on fuel filler cap threads

[JazzyB]

I was filling up with fuel the other day and I noticed rust marks on the threads of the filler cap? Has anyone else noticed this? Or could I be the first?

[olduser1]

You may be the 1st to notice.
The deposit can arise from the tank of the vehicle or from the fuel.
Suggest you undetake acouple tests to identify the source.

The fuel filter should prevent any material into the sytem downstream of the tank.

Change the filter when due on  the maintaince schedule