There is dirt and stuff embedded in the base of the board... but you'll never clean it all... you'd have to heat it up and then try cleaning it out with scouring pads and other abrasive cleaners... even then I have my doubts as to whether the base would ever be the same.
The problem is that you rode over that stuff... meaning that it was jammed into the base of the board.
I heated the board up with my iron and also heated some leftover wax on the base and pulled the wax off with a towel before it cooled... Took out alot of dirt... So it worked.. Still kinda dirty but not as filthy as before...