For spray-on hardening, I haven't heard of anything "officially," but after reading this question I had this really strange idea... you could buy a plastic spray bottle from the dollar store, mix up a batch of slow-curing casting resin to put in it, and spray it on that way. Of course you'd ruin the bottle, and I have no idea what kind of results you might get, plus I'm sure you'd get resin EVERYWHERE... Hey, I didn't say it was a good idea.