a plain old pressure washer.

if really stubborn try the Proseco line of stone cleaners.
pricey but top of the line products.

Thanks you two. I'll recommend it to the client. I was afraid it would involve muric(?) acid or some type of sander/grinder.

Don't use acid on limestone.

I wasn't going to. It's one of those 'I know I don't know' situations. I do know limestone is pretty soft, as rocks go. So acid wouldn't be a good thing.

Actually the real problem with using acid, is Limestone is calcium based, so it is alkaline, so poring on acid and you get the old vinegar and baking soda volcano.... Messy and very destructive to the stone.

A pressure washer and a stainless steel wire brush should take care of almost any stains.

Good luck,

Hap