I bought mine few years ago and the one im using right now is my second one and i love using it and like what TP said, it has some kind of wax on it that attracts the dirt,
I also bought a small one for the interior and it works same as the one i use for the exterior.
but never, ever, ever use it on the top unless you want a red furry top
and also it doesn't need washing, just replace it when it get's really dirty...