theory11 — Magic Tricks & the World's Finest Playing Cards
Or if you don't want to order online you can pick up roughing fluid at any hardware store, only it's not called 'roughing fluid'.
1260 DULLCOTE hobby/model/craft lacquer is what I have here... I've made a couple invisible decks with it, they've lasted a few months now and still work reliably.
Just be sure when you apply it not to use too much and also wait untill it dries before putting the card together, otherwise they will stick like glue.
Thanks! Now I won't have to wait for it to come in the mail. Roughing fluid is what I meant when I said spray.