[chris vickery]

I believe the Gillespie to be an ordinary alkyd enamel which can be thinned with all kinds of stuff. I usually use zylene or medium enamel reducer at a rate of about 10 to 20% reducer to 80-90% paint.
As others have pointed out, it is a balance between paint thinness, air temp, humidity, equipment, pressure etc.
I always run a part pot through the gun on some test subjects first to see how it flows, adjust paint flow and air flow as required. If I can't get it to spray nicely, obviously it is too thick.
Reducer plays an important part dependant on air temp. For summer time and not too hot, usually medium works best.
I think my next gun is going to be a HVLP (high volume, low pressure) set up as you waste far less paint due to overspray and can get away with low PSI from your air source.
Another point is to use an oil free airline with a water separator, especially if you do not have an air dryer on your comp.