There are several things you could try:
1. As you suggested a different brand, maybe an acrylic varnish.
2.See if Humbrol, or another manufacture, do matt varnish in a rattle can.
3. If you have an airbrush, which I'm assuming you haven't, try spraying it. One of the cheapie external mix air brushes would probably be fine area work like this.
Karl
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