What type of paint are you using? If it is el cheapo craft paint like Apple barrel, Plaid etc.
I heartily recommmend thinning it with Future Floor Polish. You will be amazed at how Future enhances this type of paint and its application through an airbrush.
If you don't have a product you can thin with Future then just glosscoat the finished kit with the Future. Future is self leveling and it should be smooth if not applied so heavily that it will run. Then if you need the finish flat then spray again with a good quality clear flat.
I am building a large scale refit (6 feet long), because of the amount of surface area I am using Rustoleum for the basecoat and I get some overspray. I wet sand with 600 grit and it comes out so smooth I have to rough it back up a bit when when I airbrush details over it.