Well, I'm not sure what "flat varnish" is. I've always used that term with wood stains.
What I do know is that Gunze, Testors, and Floquil all make a clear flat liquid paint (as opposed to an aerosol can) that you can mix with color paints.. I suppose you should check if it's acrylic or enamel, first! I went to greatmodels.com and searched for clear flat and came up with about 9 different products.