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!

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.

Still a skinny split bigot.

Thanks for the thoughts, I'll keep you posted

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

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.

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