You can leave the enamel as it is, but I do prefer to apply a sealer coat. I use Future acrylic floor coating. You can brush it on or airbrush it. I also use it in modeling, as a gloss undercoat for applying decals. The gloss acrylic provides as smooth a surface as possible, eliminating the tiny pits on a matte surface, that trap air, and lead to silvering of the decal film. That's also why we use it for clear pieces, like aircraft canopies and other windows. Future fills all the tiny scratches and other imperfections in the surface, making the piece clearer. You can even use it to repair a clear piece, by sanding out scratched with successively finer polishing cloths or pads, and then applying Future.
It's also cheap and a bottle will last for years, though eventually it might start to get a little viscuous. At that point, return it to your cleaning supplies under the kitchen sink, and buy a new bottle.
The name of the product has changed over the years. Matt Swan, who maintains a scale modeling blog, "Swanny's Models", devotes this page to an explanation of Future's history, and the various labels and names used in countries around the world:
http://www.swannysmodels.com/TheCompleteFuture.html
That page might be the most-cited/shared page in all of modeling.
Future is self-leveling, and if it should pool or if you get drops on your piece, you can use a weak ammonia solution (eg, Windex), to dissolve it, and then re-apply the clear coat to the area.
I find that it imparts a depth or a richness to the colors below the clear coat. I've been using it for over 20 years, and I have not seen any change in the clear coat, either, no yellowing or anything like it.
Hope that helps, prost!
Brad