it's been my exerience that you may want to use the testors acylic thinner with their paints. Tamiya thinner messes them up for me. I just recently started using a 50/50 mix of tamiya thinner and isopropyl alcohol with my tamiya paints, they work a bit better.
I haven't had toomuch luck with flowaid, though, i've only played with it a few times.