I can't answer specifically in re Tamiya Flat black and Future but, I use a lot of Craft acrylics and often use Future as a thinner, I often mix more than I need and store the paint mixture in small plastic squeeze bottles. I have some that are well over a year old and still spray and work just fine. I usually add a couple of drops of GAC 200 to each ounce if that might make a difference. My best guess is that your premixed paint should be fine if stored in a tightly closed container but then, I never actually tried your mixture.