I do a lot of dominos, I love them! :lol: I only seal the top of the domino where my art work is. I use Acrylic Spray Sealer(Matte Finish) made by Plaid. I do 3 thin coats from at least 10 inches away, it dries almost instantly although I do wait 10 minutes in between coats. It is available in most craft stores. It is the best thing I have found, no bleeding. You can also get it in a gloss finish too! A can lasts forever.
Hope this might be of some help!
BJ