I'd be inclined to start by using PPG's Amercoat 65 as your thinner and omit the R6K 30, despite what the distributor said. Maybe @Redux or @NACE could shed more light on the chemistry differences between the 2. I know that Sherwin's R6K 30 is MAK, whereas PPG's Amercoat 65 is comprised of ethyl benzene and xylene, but I don't know enough to tell you whether or not they could be used interchangeably in your situation.