First, if you are using an airbrush it is absolutely critical that you make sure the paint is thin enough to flow properly. You said you didn't add any thinner, which is only okay if the paint is specifically formulated to be sprayed out of an airbrush. From what you have said about trying to smooth things out, it sounds like you are getting a rough coat which would seem to indicate using paint that is too thick. I think your best bet would be to use rubbing alcohol to strip it off and start over.
Because it is so hot up there in Idaho, another possibility is that the paint is drying before hitting the surface of the model, especially if you are holding it 18 inches away from the model! Acrylics are notorious for drying very quickly, so to counteract this you may want to buy a bottle of acrylic retarder at an art supplies store. Just follow the instructions on how much to add, in my case a single drop is good for a small bottle of paint, half full. Anything more than 10 to 15 percent tends to make the paint too sticky, even when dry. Also, when spraying with an airbrush try to move in a bit closer, when I was still using my single action Testors airbrush (with canned propellant, but
without a regulator!) I worked at most a foot from the surface.
When you thin acrylic paints, the cheapest option is to just use pure rubbing alcohol. You can also use window cleaners like Windex but they contain other chemicals like ammonia that may have a detrimental effect on the binders in your paint. Many people use complicated ratios to determine how much thinner to mix, but I just add thinner until the paint has the consistency of milk. I always make sure I don't thin it too much by dragging the stirring stick up along the side of the bottle, watching to see what the trail of paint it leaves looks like. If it is so runny that it quickly drains back down without leaving much color on the side of the bottle, it is probably too thin and you may want to add a little more paint. Always test out the paint on a piece of paper to two before spraying it on the model, it will spare you from having to strip the paint off later.
Should you ever come to own an air-compressor like what my dad has for his pneumatic tools, you can easily convert it for use with an airbrush. All you got to do is do that project or two that the wife has been bugging you to do, and use that as an excuse to buy a large compressor w/ tank and some air tools. Then you will be set for life...