Try the new Krylon Acrylic Clear. I've had excellent results with it over a number of paint brands. I painted an Oulander with Testors flats, used the gloss clear to provide a good base for decals, and then used the matte clear to restore the flat finish. Worked very well. There are a number of formulas, but the latest I bought is marked UV Resistant.

Here's the Outlander. No visible edges of the decals, like you'd get normally if you applied them over flat paint.

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Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp