Can someone recommend a good brush on matt varnish? I'd want something that could safely cover both acrylics or oil paints.

Humbrol?

I have not heard good things about Humbrol's matt varnish.

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.--

Brush on matt varnish

I agree with Steve,
any varnish will play up now and then....

I didn't stir/shake/mix a bottle well enough once, dreaded white spots everywhere...

I'm am using Vallejo Varnishes, satin, gloss and matte, with great success so far.

As always, the lower layer needs to be completely dry, and while acrylics dry quickly, oils are an unknown to me so far....

HTH

Cheers

Matt Varnishes

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.

Brad spelts

Try Humbrol clear flat, just mix it well. A thin coat works best.