I’m glad I’m not the only one with a gelding who has an unusually dirty sheath. I wish I had advice for you however, 
but I’m in the same boat.
I’ll share my story, sorry if it gets long, but this has been bothering me for years now and neither I nor my vet know what to make of it.
My morgan gelding gets thick gooey smegma built up very fast too. Not on his penis, his penis stays relatively clean, just the entire ‘cave’ of the forward section of the sheath.
He has apparently had this problem his entire life, and with his former owners his sheath was never cleaned, so its not a case of overproduction due to over cleaning. When I first got him, he had a rat’s tail from rubbing, and the tip of his penis was always sticking out. Wasn’t until having him for about a month, I went to stick my hand in there to check things out, and discovered his sheath so thick and full of caked on smegma it was falling out in clumps.
Not crusties, not flakies, thick heavy clay-like goo. Imagine taking a handful of brownie batter, smearing it around in there and letting it dry a day or two. I’ve never heard or encountered such a thing before.
When I got the first layer off the goo off, he was so relieved. When I went to the bottom layer, he started to kick. I gently inspected, and the smegma had been built up for so long, it was almost attached to his skin and the skin was cracked and tender spots. Poor baby. It took me a week of small amounts of gentle cleaning to get most of it out.
Since then, I have restrained myself to weekly picking of clumps and only cleaning when he begins to tail rub, because I do not want to stimulate more production with over cleaning. He gets cleaned about 4x a year.
He is very very very grateful to have his sheath cleaned. Though my boy does have very muscular ‘hams’ and bugs and grit does get caught in the folded skin from the back of his sheath up to under his tail, I’m pretty much convinced that the 90% of the reason he tail rubs is his sheath. He’ll go seek out bushes to stand over and itch his sheath, and I have to make sure I hang his hay bags high and tight, otherwise he squats on them and uses them like a loofa on his sheath (I have a photo someplace, I’ll dig it up).
Anyhow, I have not found out what to do about this problem. My boy is a dark liver chestnut, he is thin skinned and extremely sensitive overall. He is IR (and I sometimes suspect PSSM or very early Cushings) and I have to manage his diet very carefully.
He does get daily oil, cocosoya and wheat germ, it keeps the scalp under his mane and tail from becoming extremely dry and crusty. I do often wonder if the oil is contributing, but then I remind myself he’s had this problem since before I got him. And in his former life, he lived out with cows for years, basically ignored, eating grass with the cows and hay in the winter, no grain ever, no supplments, etc.
I don’t use Excalibur, there is something about it that I think stimulates more production, and its not quite strong enough to cut through this thick goo. The best I have found to use is Novalsan scrub, but its ungoldly expensive, but it does cut the grime without being harsh. I usually use baby version Castile soap, which works well too (but then I hate the slightly floral scent that lingers for weeks, it grosses me out). I was once desperate enough and used Dawn, it did work but no better than more gentle Castile.
My boy’s production seems to be triggered mostly by the skin getting irritated or dry, but I also think from any attempts at scratching. In the winter time, since I rough board and have no hot water or warm and dry barn, just a run in, I clean with mineral oil and though counter-intuitive, it actually does a good job. When I clean with soap in the summer, if he feels dry, I slather coconut or jojoba oil inside. Oiling the skin seems to buy me an extra few weeks, so I can clean less.
Really, I restrain myself, I know over zealous sheath cleaning does more harm than good.
In the end, my gut is telling me my boy’s problem is somehow diet related, but I can’t figure out what or why.
Sorry for the novel, this has been bothering me for years and I still just don’t know what to do about it.
