Which paint to use.

Hey everyone, I've been modeling for years and recently just got serious about it. I have an airbrush and basically only use it for the big parts, ie. fuselage halve, wings, and such. I'm satisfied with the way my models turn out with one exception, the decals.

I was wondering if anyone could help out, I use the Testors paints that you can get from anywhere, they were given as gifts so I might as well use them. The trouble I'm having is that I need a glossy surface for the decals to look right, but I normally need a matte finish for the model. So if there is any product that I can use safely with these types of paints please let me know. I've tried unsuccesfully before and it only ended making the model milky white. thanks for reading.