I've written before about improving commercial matt varnishes. Despite this there are still occasions when I have the odd "less than ideal result" usually just before I apply one of the fixes I've uncovered over the years.
One such occasion occurred this morning whilst varnishing some 15 mm Swiss halberdiers for my renaissance French army. The problem was only obvious on the red and blue jackets of the figures which retained a slight satin sheen.
Experience has taught me that another coat of Matt Cote doesn't help and, for some reason I can't fathom, neither does using other varnishes like Windsor & Newton Acrylic varnish. I know some of this is caused by the nature of the final paint layer (all Vallejo in this case) but I usually get around this by matting susceptible paints with a touch of talc - see Painting Tips #5 & #5a. In this case the paint layer was a true matt so the problem must be with the varnish mix: it's now been diluted with a touch of white spirit ready for next time.
Even so the Swiss still needed a true matt finish. By far the best way I've found to date is to use Flat Future. This mix of Tamiya Flat Base X-21 and Future floor polish is well known in modelling circles and I followed the recipe given by FichtenFoo for a Flat/Dull finish:
As I wanted to "repair" or refinish the figures, I simply painted the 30:70 mix of X-21 and Future on to the affected parts. I used the Flat Future mix sparingly, almost wet brushing it on, to avoid it pooling. If used in excess the "super flat" mix I have at the moment can leave a white residue in folds, and silvery flakes on flat surfaces. This is why I don't use it as my varnish of choice.
Its main advantage is that it dries really quickly and you can see one portion dry matt before moving on to the next; even on 15 mm figures. The strength of the Future layer is a nice additional feature. It drops the colour intensity a notch but that's to be expected as matting anything reduces the light reflected from the surface.
Let me know if this helps you.