I've always achieved the results I was looking for when using 70% strength rubbing alcohol as a thinner for spraying Tamiya paints. Rubbing alcohol has just the right mix of denatured alcohol, water and isopropyl alcohol to thin Tamiya paint without causing it to dry too soon. As an added bonus it's pretty cheap. If you still get the "orange peel" effect when using rubbing alcohol, try using less thinnner and/or reducing your spraying pressure.