If you are talking about what looks like basically a buildup of dead skin on the front of the cannon bones, then I know what you are talking about. My gelding has had this ever since I got him at the age of 19. I have never really seen him get pee on him, and he usually waits to pee on the sand or in his stall because he really doesn't like to be splashed. I groom him every day, and I clean everything to well....sort of a neurotic degree ;)
I once asked the vet about it, and they basically said that it is a rather common form of dermatitis. I have tried everything on him to no avail. However, my mare, who did not have it when I bought her, somehow contracted it from him (likely from the brushes- I no longer share brushes). I was successful in getting rid of it on her by using MTG on her legs every evening. Just be careful to wash it off after a few applications otherwise it can irritate the skin. I treated my gelding with the same stuff, but I think that it is just so established on him that it's impossible to get rid of at this point. I also scrubbed and brushed the legs every day and then used bleach water to sanitize the brushes afterward. Also, no sharing brushes at all.
My new gelding did not have it when I got him and still doesn't have it now. I keep all their brushes, boots, and polos separated, and this seems to do the trick. I definitely do not think that it is environmental at this point, nor do I think that it is caused by urine.
So, just thought I would share my experiences. I would recommend trying the MTG though, because it was the only thing that did work on my mare.