A rough, pebbly finish is usually caused by the paint drying (or partially drying) on its trip from the airbrush to the surface of the model. This is frequently caused by painting from too far away which, in turn, is frequently caused by too much pressure. Too much pressure, not keeping the airbrush constantly moving, and / or spraying too close to the surface will all cause the paint to run or pool because there is just more paint on the surface than it can handle. Liken it to a piece of window glass in winter. Breathe on it and you get fog or mist. Throw a glass of water on it and you get runs.
You have to adjust your air pressure and distance from the surface based on the amount of paint flow because the surface can only take so much before it will start to run. You also have to be close enough that the paint doesn't start to dry before it gets to the surface. Thin paint will spray at lower pressures than thicker paint (I know, a generalization but usually a true one). Also, several light, even coats are much better than one thick, heavy coat.
I like to spray relatively thin paint (Tamiya acrylics 3:1 (paint:thinner) thinned with isopropyl alcohol) at about 15 psi from about 3 or 4" from the surface. Not having a regulator means you'll have to play with your bleeder valve until you find something that works for you. I used to keep a piece of styrene sheet on my bench. When I wasn't sure what paint viscosity / air pressure / distance to use I just shot at it until I found what worked.