I have the black and white matt rsvr and when I first got it, the dust and dirt would become engrained in the white tank paint, no matter how much polishing, nothing would get the dark patches out, in the end I venture shielded the sides of the tank in clear invisible plastic where my knees rubbed.
Solved the issue, now it's always cleanable and mint, I did think about having the tank clear coated, really did nark me that the paint is so porous and prone to absorbing dirt.
Hope this helps.