cant really be sweat/dirt imho.
I was basically wiping it all day long with a REALLY wet towel, and as soon as it was dry again I could draw stuff on it.
The water-proof surface was not working anymore and the speed was gone aswell.
also, I wont wash my pad in the machine, not worth the risk
I did it with the old razer mantis pads, mine was stained by coffee and other stuff and basically was locked up in a drawer for 2 years till I once again used it.. I then decided to wash it in the machine and well... it was clean afterwards, but the friction had become so high it was unusable...