stable stains?

My piebald mare that I share always has stable stains on the back legs from the mud! I can get it to fade a little by scrubbing it with a rubber curry-comb sort of thing with long fat rubbers bristtly things on but that's it! I can't use water or any chemical so is there any magical tricks anyone knows?
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I use a bot block/pumice stone which works wonderful for me.

Great! Thanks will try this!
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Unless you are going to a show or a parade, what does it matter? The horse is ok with it. If you fuss with it too much his skin will get sore.

Well, the thing is, I'm new at the yard and I don't want it to look like I can't groom propebly.
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