I'm a big fan of PolyScale flat finish; in fact I use the PolyScale range whenever I can. Great stuff.
There's another, extremely simple technique for turning glossy decals into flat ones. (I can't claim credit for this idea; I read it on a kit instruction sheet - a Monogram one, I think - many years ago.) Before you remove the decal from the backing paper, rub the surface of the decal with a soft pencil eraser.
I don't know whether it works with every brand of decal, but it works on the ones I've tried.
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