Steve,
Polly Scale is very good. Polly Scale makes a thinner for thier paints it's
#546008 in an 8oz. bottle . It retails around $6. I use sodium free distilled water
in the gallon jug that usually costs $1 dollar or less. It's cheap and also works
great for decals and airbrush cleanup immediatly after use.
When I thin Polly Scale paint I start with one SMALL drop 10%of water and
that's usually enough . I also keep a damp paper towel to clean the airbrush tip .
For me it's worth every penny .
Harry