MEK is methy ethyl ketone, which has one more carbon in the chain than acetone. So its solvent character is the same, just less volatile and slower to "Flash Off". Styrene is the monomer from which polystyrene is made, so when mixed in as a thinner it will become part of the polymer. Caustic? I don't get that part: There is already a lot of styrene monomer in the unpolymerized resin in the can. So you don't want to huff it or breathe it unnecessarily.