See John (Yankee Modelworks) Sheridan's review of WEM Colourcoat paints including a comparison of several different thinners
http://www.steelnavy.com/wem_colourcoats.htm
I've found that the paints handbrush very well when properly mixed without thinning.
However for airbrushing, thinning is required. My thinner of choice is lacquer thinner from the local hardware store (12 dollars a gallon versus 7 dollars a pint for one with a model manufacturer's logo) .