My mare has a very thick black tail that is about a half inch from the ground. I love her tail it can take me about an half an hour to fully brush it all out (I only fully brush it all out for shows) But since she's a mare it gets kinda gross and stained. I haven't really found a way to get all the crud on the underside out of it.
Are you asking how to get the "crud" out of her tail? If so, I would either give her a bath and bathe her tail really thoroughly, or if you don't feel like doing that, you could just use your fingers and pick it all out. The latter is what I do in daily grooming. Posted via Mobile Device
Anyways...My mare a a pretty dark reeeeally thick long tail as well and I hate to break it to you, brushing it out only before shows doesn't cut it. I would wash and brush my mare's tail out once every week-two weeks at least! All year long of course. If you're having trouble with lots of stubborn "crud", I would shampoo it really well and let it soak for a little bit. What kind of shampoo are you using? I can probably recommend a few different ones.
My Palomino mare has a white/yellow tail, which should be totally white but due to the urine stains is more yellow on the bottom...ick...but I have shampooed it with some success (used that blue wonder shampoo whatever it is). Next time I plan on trying one of those shampoos specifically made for removing urine stains on white tails. Not sure what to tell you for a dark tail though, sorry.
As far as brushing out the tail....my mare's tail is pretty thick as well and after shampooing and during some grooming sessions I spray Laser Sheen through it...rub it through with my hands, wait a few minutes, and then brush it through and it REALLY helps lots with the tangles - seems I can brush through her tail in no time using this, plus it leaves it smelling great and very shiny!
I leave her tail alone because that's what keeps it so long and thick, instead of brushing it everyday and pulling out the hair. And bathing all the time just strips the natural healthy oils. Could easily make her skin itchy and irritated. There's no major tangles in it and I can run a brush through it all the time, I just don't mess with every layer. And yes dark tails get urine stained, it turns the hair into a nasty brownish yellow color.
That's my mare's tail. I PROMISE you, regular bathing doesn't not break the tail. If you are worried about the oils in the skin there is a product called quikclenz that was used every other day on 90% of the horses at my barn because it's safe and does not strip oils. If you still don't want to bathe regularly, you can wash just the tail. With regular shampooing with a product like Cowboy Magic, Mane and Tail, Quikclenz, even many "people shampoos" can actually help strengthen the tail. Cowboy Magic also has a detangler, so does Peppy, Showsheen and a few others I can't think of off the top of my head. These I've found work liberally on a freshly washed or at least rinsed tail.
ETA: I didn't say to brush everyday, I said once weekly. Once ever two weeks will work too on a tame tail that has been worked with. My mare's tail stopped feeling so hard like a horses tail usually does when not washed properly and became quite easy to brush through in just 10 mins when I kept up with my routine.