I've had several problems with older dry-film resist, but the worst was that it was extremely difficult to get an exposure that would protect traces without also bridging fine detail and traces.
The thinner I prefer for diluting the Chinese liquid resist is acetone since it dries quickly and is fairly cheap, though similar polar solvents like M.E.K. or even denatured ethanol work. The auction page for the resist suggests using "banana oil", but that solvent isn't terribly common and is quite expensive compared to garden-variety solvents you'd find in your favorite home-improvement store in the paint section.