It kind of depends what type of paint you are using.  I use lacquer, so a final coat of gloss or flat clear lacquer locks everything down.   If you are using acrylic paint, you will need to use an acrylic varnish.   Aqua Gloss, Vallejo, etc.   I find those a little tricky to get a nice even gloss without runs, but it certainly can be done.  Don't build it up too thick or it tends to yellow.