I have a cotton yarn sling that I’ve been thinking about waxing. Has anyone tried this before? The braid is fairly tight and I think it will keep it from fraying and looking fuzzy but on the other hand it’s also not really a reversible decision. I want to make sure this isn’t a really bad idea before trying this. Anyone have any experience in this area?

Haven't waxed cotton myself, but I imagine it would be better waxed than un-waxed. Cotton frays easily, and wax would really help add durability, but because cotton isn't a stiff fibre, if it was waxed it shouldn't be too stiff.

I really like waxed wool, as long as the elasticity has been removed beforehand. Waxed wool is really good in all weather, its very very durable, and it still remains 'floppy'.

As long as the sling is not 'thick' waxing works well.
I wax mine by dipping the sling in liquid beeswax, and then run my hands down the sling to rub off the excess (wearing rubber gloves) while it is still hot.

Hope this is helpful.