That arm looks as though its had the dreaded cover-up gunk slapped on it, some people will say its protection, but who is it protecting?
This is why I'm not keen on covering up such areas with the black stuff, I understand its good to treat surface rust, but it has to be done properly when the car is still a few years old.
There comes a point when surface rust has gone too far, so all the owners end up doing is hiding these potential dangers from being picked up.