Hi..

Toluene solvent is good for thinning PDMS. I have used it for getting
PDMS films down to submicron level with extensive dilution of toluene. I
guess, it would be ok for dilution of polystyrene too. Probably, you
could do the surface prep before spinning on polystyrene. Like cleaning
surface with methanol and then plasma activation and then do immediately
coating of polystyrene.