Tamiya's bottled primer has a lacquer-like base and can be thinned with regular lacquer thinner. However, this may be a little aggressive on models. You can use "hobby branded" lacquer thinners like Tamiya's own lacquer thinner or Gunze's Mr Color thinner, which are both formulated to work on styrene without being overly aggressive.
I have also found that you can thin it (and clean your brushes) with 90%+ denatured alcohol (ethanol). It will not work with Isopropyl alcohol. Having said that, I have only experimented with airbrushing it, just to see if it would work. It did work, but I can't suggest any particular thinning ratio - this will vary depending on your airbrush and pressure setting.