I've been thinking more and more that if used properly, skinny split pouch slings would be the most efficient.
I do find these skinny split pouches let the rock slip out more often and was wondering what you might suggest as the grippiest material to line the pouch arms with.
So far I'm thinking spray on rubber just enough to speckle/pebble the surface

Rocks are flying out even with leather coverings? If that's the case you may need a larger pouch or softer leather to cradle the rock better.

& to answer your question a grip spray would probably be your best bet. Something like Seymour's would work well, though my main concern would be a loss of flexibility on the sling once it dried even with light coating. Good luck!

medical tape works pretty well.

But the actual answer is a solid pouch :-)

I've found skinny splits are very ammo size/weight specific ... too small or too light and the ammo will crawl out at the earliest opportunity and not wait for your release.

Skinny splits are also sensitive to how fast you accelerate the sling in your wind-up. Accelerate too fast and the ammo can stretch the split slightly and fall through. :'(
I saw Jax do this with a baseball sized sling. I could throw fine with it but Jax's acceleration was so great the ball would pop through like a button through a button hole. :o

Still a skinny split bigot. 8-)

What do you mean by skinny split pouch? 90% of my slings are split pouch , i have no major problems. 
Occasionally tiny smooth stones will slip through trying to tear the end of the split pouch ,or strangelly shaped and badly positioned will fly off. But i dont prefer tiny smooth little stones .

On trying to make a better sling : try it, experiment(cause its nice) but at the end of the day the simple old natural fiber wins. I've found that the more complex it becomes to make it, the worse it will be.

Thanks for the thoughts, I'll keep you posted

@Parmenion
A thin round or flat braided sling where the split pouch sides are the same cord size as the slings cords 
pretty much what you posted here
http://slinging.org/forum/YaBB.pl?num=1446140154/8#8

I'd say the issue is probably your form/windup, not necessarily the sling itself. (unless you're using a super skinny split-pouch with a split 6 or 7 times longer than the ammo you intend to chuck with it. Then it's possible your ammo is just ill-suited to that sling.)
While you probably could coat the sling with something super grippy, you shouldn't need to. Work on your form instead. While some sling designs may be more forgiving than others, jerky windups with any sling rarely go well ;)

Bees wax is grippy. Even on a sissal split pouch. I agree with Donner. Pouch length is important. A five inch pouch is plenty long for something as big as a tball ;)

I agree with donner in that its probably because of your form, i've had a similar problem with some of my tennis ball slings. It's all about having a build-up before the release so that the ammo isn't just "forced into the pouch" instead its slowly put into the position that its supposed to be in. So just fire a little slower for a little bit then find a way to add force to the ammo without just throwing as hard as you can. :) hopefully that helps.

As the other's said, pouch length, form, and ammunition weight may play a more important role than texture. The only time I've found texture to be an issue was with some kinds of plastic twine, any leather or other fibers worked just fine. Personally, I'd go with tarred bank line and maybe some medical tape. Slings that are too light just don't feel as good to me when you throw, especially the lower resistance ones.

Good luck and post some pictures when you finish!

I also agree with Donnerschlag. He hit the  nail on the head I believe.