Static Electricity
Why a page on static electricity?
Because static attracts dust. And no matter how smooth your paint job is, it won't look good with dust in it. The blast from the airbrush may blow some dust away, but probably not all of it.
In this first picture, a piece of plastic sprue (on the right) was rubbed with a piece of T-shirt and then stuck in a hole in the Styrofoam block. A strip of paper towel was taped to a stick and brought near the sprue. When it was about one inch away, the paper was attracted to the sprue as shown due to static charge.
In the second picture, the sprue has been wiped with a towel dampened with a mild soapy solution, dried, and placed back in the hole. Now the strip of paper can be brought to where it almost touches and there is still a gap.
Plain water would also have worked, but a thin film of soap left on the plastic makes it less likely to build up a new static charge from light wiping or handling.
This shows one reason why it is a good idea to wipe down a model before painting. Not only will static charge be reduced, but any grease or mold release on the plastic that might interfere with paint adhesion can be removed.
I make the soap solution just concentrated enough that it foams when I shake it. I have had no trouble painting over it with ModelMaster enamel, but you should try it on some scrap plastic with the paint you use to make sure. If it causes a problem, just wipe it off with plain water.
This also works well on clear parts that are to be dipped in Future.