From the manufacturer's website:

This acrylic varnish is truly unique and has been tailored to remove the shine of any models being dipped with Quickshade or painted using conventional paints. Although really matt, the sprays original formula retains the metallic shine from Plate Mail etc.Always let the Quickshade dry for minimum 48 hours before using this varnish and apply it in a thin coat. Never spray in humid conditions and cold weather. Test on a model first before spraying the whole army.