This may be the result of the high humidity you report. Do you have a moisture trap properly located in your air brush line? If not, that may help the problem.
If you do have one, make sure you blow it clear more frequently in humid weather.
It is also possible that the Gunze coating contains hygroscopic components that are "grabbing" moisture from the air and causing the problem. Increasing thinner ratio slightly and increasing spraying pressure may also help. If not, try the opposite approach.