The thinning medium is Plaid paint floater. You can get it at any craft store that sells the large bottles of craft paint. You could also use future floor wax.
I wish I'd found this two weeks ago. I just went through a long and annoying process of experiemntation and wound up with an alomst identical technique.
great article! I love the final result and you didn't leave behind any important aspects. Everyone could read and understand this article. Thanks! -Erik