Model Master Acryl Paints

The only reason to thin ANY paint is to get it to spray at a particular pressure. If a paint sprays right from the bottle at the pressure you want to use there is no need to thin it, if not then it needs thinning.

For MM and Tamiya acrylics I prefer to thin the paint slightly and add some retarder to slow the drying time down. I also prefer to spray at about 10 psi so I normally need to thin my paint.

Another thing to consider is that event within identical colors from the same paint brand, you might see some batch to batch variation. In on extreme case I experienced, a bottle of ModelMaster Sand that I had needed a typical amount of thinning prior to airbrushing whereas a replacement bottle of the same color and part number sprayed perfectly without thinning. The bottom line is that there are no hard and fast rules that you can follow other that to develop through experience the judgement of how thin your paint needs to be for airbrushing and then thin your paints accordingly.