I see, that explains everything.
I would never criticize anyone for choices made to save money, and I will not do so. In fact, with a little experimentation, you might get that artists paint to work. I've used paint like that (Liquitex) on war game figures before.
But here, it's fighting you and causing the problems. It's made to be very gel-like, and usually in fine art brush marks are ok, even desirable. On a model, not so much, right?
Whether or not you invest in an airbrush, familiarize yourself with quality model paint.
The little Testors square bottles of enamel are really great. They thin out well and hand brush just fine.
Hope that helps.
BTW it's good you posted this. Hopefully you'll take a leap in results and like what you create.
Bill