So, I decided to find an alternative breath weapon for my dragons, that maximizes destructive potential for the least amount of weight. That's how we arrived at aqua regia, an insanely strong acid that could still be synthesized by a biological creature.
As it turns out, neutralizing an acid, that ate Star Platinum and Jotaro in one sitting for dinner, comes at a cost, namely, mustard gas. That's rather problematic, since dragons are exposed to that and aqua regia, whenever they use their breath weapon.
So, I guess it'd be better to keep away or stall aqua regia. This defense has to
Protect the dragon's skin.
Be translucent, as it has to be applicable to the nictitating membrane.
Dragons should be able to produce a special, protective saliva as well. They lick their wounds to prevent them from being contaminated by the chlorine gas and the occasional aqua regia droplets.
What biologically possible material, that can't be scraped away easily, could stall or maybe even stop aqua regia and its byproducts?