I personally recommend Krylon Matte Finish in the spray can, used in light misting coats will remove any shine. I use this to flat finish all my models. Never had any problems or mishaps with it. Does not interact with Future at all.
You’ll love the Pollyscale Flat…best flat coat I’ve ever used! I cut mine about 15% with distilled water for airbrushing, and I was amazed at how well it worked.
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.
I’m gonna try the Krylon Matte Finish first because i can get it today! $2.97 at Walmart.
The Model Masters and Polyscale I will wait until I can place a larger order from an online hobby shop. My local hobby shop and Hobby Lobby are always out of the MM flat spray and bottle. Neither carries any PS products. They have lots of gloss though!