RX-20 leaves black spots

I ran into a problem with the RX leaving black spots when leaving it upright during jobs to move the hoses or whatever. Come back and I notice a few dark spots. They tend to come up with a few passes but it gets annoying.

Has anyone had this issue before and was it just dirt clogged up inside the star or was it an oil type leak? Maybe too high psi??

Also, what methods do you guys use to clean your star? Pressure wash it or what?

Thanks guys