Mike,
I have been using for years, Xtracolor flat & Testors Gloss coats.
They are both lacquer products though, so I usually mist several fine coats so they won't react with any enamels I have used for the paint coats.
Another product I really like, is MRP. Another lacquer, but I have had good luck with their clear coats as well.
I never got the hang of acrylics myself, so use they occasionally in my builds.
Larry