Here's my original post on this subject from 1-23-09. It's based on what a local vintage restoration shop does, and they've worked miracles on the two 118, one 215 and one 415 cabs I took them. The reason it's better (but not life-or-death) to take the cloth off the cabinet is so you can clean it all evenly.
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To clean the grill cloth, get some Tuff Stuff (made by Simonize) at an auto parts store. If you leave the cloth on the frame, spray it on VERY LIGHTLY and wipe off with a towel. However, the best way to do it is to take the cloth completely off the frames (pull all the staples out first), spray on the Tuff Stuff, scrub it with a nylon brush, then blast it with COLD water at a car wash or use a pressure washer if you have one at home. (Hot water will shrink the cloth, creating new problems, so don't do that.) It takes a lot of patience to get the cloth back on the frame straight and taut but the result is worth it.
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Good luck!