Greg
Right, don't mix Mr Leveling thinner with Tamiya Acrylics.
Actually, that is preferred for airbrushing Tamiya paints. It makes them go on semi translucent and silk smooth. Its great for shading and weathering, but yes, I wouldn't use this combination for brush painting at all. Use some 70% alcohol or Tamiya's own X-20A to help it level out for brush painting.