Airbrush Surface Prep
The airbrush paint job will be only as good as the surface that it goes on. You will not hide unevenness, cracks, wood grain or anything else with the paint.
The most critical ingredient in getting a good surface is sandpaper.
Use various grades from coarse (220 grit or so) down to fine (as small as 800 grit). Do not use the bare sandpaper with only your hand, as this will leave unevenness in the surface.
Get sanding blocks for the hardware store: I like the ones that allow you to remove and change the paper.
The open grit dry sandpaper works well for most of our purposes, although I also like to wet sand after primer coats.
If necessary, apply fillers to close up bigger gaps or holes. Sand with 3 or so grades in sequence, until the surface is smooth to the touch. Look at the surface by angling it upwards to a light.
When you think you are done, sand some more. Use a dust mask throughout this whole process.
When the surface is smooth, it will need to be cleaned before painting. At this point, you should only handle the model with disposable plastic gloves (wear these when you paint also).
The dirt and oils from your hands can affect the paint quality. Change your gloves from time to time; wash and dry your hands before pulling on new gloves.
If it's fiberglass or fiberglass on foam, you can clean the airplane with alcohol. For wood or bare foam, I use a tack cloth from the hardware store. This is great for picking up dust.
Then use a vacuum cleaner with a brush attachment, go over the whole model or part again. Then tack cloth again. If it will be a while before you can airbrush, store the parts in a dust free area, covered in plastic. The message here is that dirt and dust are your enemy!