/ Cleaning holds
Stick them in the dishwasher. Really, it does work.
power washer will bring them up as good as you can get them.
If it's only a few that are dirty a sink of hot soapy water and a scrubbing brush will bring them clean.
We use this stuff at the wall I work at http://www.diy.com/nav/build/building-materials/building-chemicals/cleaning/Cementone-Brick-and-Pati...
Just leave them in for 30 seconds or so then give them a scrub and a good rinse. Dissolves all the chalk and will get most rubber off too with some scrubbing. Waaaay cheaper and quicker than benky hold wash. You'll need a good pair of chemical resistant gauntlets though, its pretty nasty stuff to get on your skin!
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