It's many years since my airbrush experiences but I think you are using too much solvent and not enough pressure…try experimenting by spraying into a plastic cup… I saw your other post re air reservoir. If your comp does not have one you can experiment by inflating a car spare tyre and tehn connecting the air brush….inflate to 40psi. As ever, a regulator with a conbined moisture trap is more or less essential for any serious airbrush use. All good comps such as the 'Bambi' units come with reservoirs and reg/trap……these units have been, and still are, quite pricey albeit UK made items, however, bambi no do a nice amateur model which is small and silent with decent sized reservoir and comes in at about £100 or less according to supplier/online etc.

If you look online/youtube there are people putting together their own comps…..Teh Steve Jordan video is probably the best one as he makes wha tis virtually a pro tool using a fridge unit.

Always used to use Humbrol in air brush, thinned down and used multiple coats, allowing to dry sufficiently between coats. Never had any trouble with getting good finish. I can’t remember what I used for thinner, may we’ll have been turps, as I don’t recall buying anything special.

Something else to take into consideration that thinning too much will affect the paint so thin less and if possible change the nozzle size on the airbrush. Some of the cheaper airbrushes don't have this facility