A bunch of us here have used Windsor and Newton acrylic matt varnish brushed on for years. It takes some shaking prior to use, but it is as a rule reliable. My personal experience with matt varnishes is that they all act up on occasion but this one is good much more often than not.--
If you can find it, Polly-S Matt Finish works over acrylics and oils. Never had a problem with it. Also, if anyone knows a good dealer source for it, please post. Thanks.
I have had good luck with Testors lusterless flat and also with Ceramcoat matt varnish. they are both fairy inexpensive, but always try it on a sample first.