I've been heating my home lately with wood, I do it when the temp drops below freezing. It really dries out the air. The other night the paint was almost dust at the nozzel and drying on the tip. I was using Vallejo Model Air with flow improver at the recommended ratio at 18-20 psi. Lowered the pressure to 14 this time it sprayed a bit better but left a "dusty" surface/finish. Can I add more flow improver?

How far away from the surface are you when spraying? If you are too far away the paint will actually dry just before or just as it hits the surface leaving a grainy finish. The paint should lay down wet then dry shortly thereafter. I would try working closer to the surface then try adding more flow improver if that did not work. Good luck.