Robert, it's still a good idea to at least clean the model with a damp cloth, then a dry one. You can't always see it, but there's normally always grease etc on the plastic. Some of this is from the release agent used in the moulds, and some from handling. When brush painting, the paint is thicker, and 'moving' across the plastic, and therefore 'takes up' some of this grease, causing less problems. One of the other reasons for the imperfections you have experienced might be due to the thinning ratio of the paint, or, perhaps too much paint in one area. The latter is especially true if you've gone over the same area more than once, before the first coat is fully dry. One of the most important things to remember when using an airbrush, is to keep the 'brush moving, preferably in straight lines, in one direction, and at the same angle; avoid, if possible, twisting your wrist, as this will mean a different spray pattern, and consequently, differing 'thicknesses' of paint in differing areas. If in doubt, practice first. The easiest way to do this is to use ordinary ink, and spray onto a piece of card, preferably smooth surfaced, in order to get used to the brush without wasting paint.
When you are ready to paint an actual model, again, have a practice spray first, using the colour you wish to paint with, immediately before attempting to paint the model. Again, just spray a little onto card, or paper, perhaps whatever is covering your work surface. This will allow you to alter the mix if required, and adjust the 'brush/air settings to suit the job.
It's much better to use a bit more paint etc. than possibly ruin a model by rushing into it. Don't worry though, we've all been there! And it still happens now and again!
Using an airbrush is very similar to using a full-size spray gun - but harder, as everything is miniaturised; it takes practice, and it's nothing to worry about, as even professionals, artists, modellers or car sprayers, still mess-up sometimes!
Hope this helps you a bit.
Terry.