Nope ... no need to top coat.
The paint should be absolutely dry. That's the advantage of airbrushing!!
You mist the paint on, layer by layer, so that it is dry immediately.
One of the things beginners at airbrushing always do is try to get the finished depth of colour all at one go ... that's when you have troubles with paint not drying, cracking, crazing peeling etc. MIST the paint on, layer by layer, and build up that depth of colour that you want. Once you have learned this, airbrushing becomes much easier.