When I spray the Testors clear on, I use 2 to 3 light coats. I've never had any issues with it reacting to the underlying paint.
I've tried other "matte" sprays, such as Krylon, but nothing I've tried gives the type of finish that I look for.
Years ago I also tried Polly Scale matte coat and it left an almost milky appearance that I didn't care for.