Beeswax

[Brucicus]

I've also seen videos where arrows coated with beeswax had significantly better penetration than those without the coating.

I'm not sure about other materials but regarding steel armour, it has been debunked a few times. There is no noticeable difference between the penetrative properties of a waxed arrowhead compared to non-waxed ones.

Thanks Dan!  I think the target was some form of mild steel.  Various points were tested, including bodkins.  i am very probably remembering it wrong.  

Bruce