Have you tried soap and water with a REALLY good session with a rubber curry? Hose em down, pour on the soap of your choice and circular motions with the rubber curry brush to really bring up the dirt, then hose again and squeegee the "dirty water" off of em. Seems to get my girl clean.
Welcome to my world. I usually just brush Phar Lap really good before I ride and then brush him again when I am done when it is colder out. Other than that I just let him roll and have fun in the dirt.
I have decided that Isabella's legs will probably never be completely clean! I can get the rest of her looking good, but her legs will always be a dirty gray. Since I don't show her, I just ride for pleasure, it is not a big deal.
I agree with Tianimalz. If the weather isn't good enough to bathe, just use a rubber curry and put some elbow grease into it. The stains might not come out the fist time, but after a few days of that, the stains will be close to gone if not completely.
This is the curry I use:
If that doesn't work, and the stain(s) is really bothering you, then just spot clean. Take a wet cloth and sock it in some water (or soap if it's a bad stain) and rub in circles into the area. Dry with a dry tail if it's too cold or windy.
Good luck! Don't the light coloured horses always seem to find the dirtiest spots to roll and lay in? (;
Oh gotta love "greys!"
In the winter I used to occasionally get a bucket of boiling water, and dip a towel in it, wring the towel out, and "steam" out the bad spots. My grey also got staining particularly bad around his knees and hocks, and I found that rubbing charcoal into the stains, then brushing the charcoal out got the stains about 85% of the way out.
In the summer time I found that bluing shampoo worked the best (like what's in hi ho silver, but cheaper cause it's made for little old ladies to whiten their hair). Just don't leave pure bluing in too long, or you will get purplish patches!
While this stuff works great, I can't get my horse truely clean in th ewinter. Baths are a no go and he is always rolling. In the summer he gets garss stains on his knees and little yellow spots from rolling on the Buttercups. This stuff helps, but a grey horse is VERY VERY VERY difficult to get clean and stay clean.