Originally Posted by Palomine
Dirt and dust getting in pores maybe? Sun damage possibly.
Could be the heat has gotten sweat and dust to almost make a paste?
Try wiping with ACV, the Heinz brand, on a sponge.
This could be fungal, and I agree with trying ACV, but beware, Heinz is not ACV, it's apple flavored/colored white distilled vinegar. Huge difference. ACV has probiotics and anti-fungal properties, where regular vinegar is a good cleaner, but not the same thing. Whenever you use ACV makes sure it's raw, otherwise it won't really do any good. :)