I sprayed future of the model put on the decals. Now do I need to put another coat of future or can I just flat coat it
Flat away … sounds like you’re ready to go, as long as the decals have adhered well to the future.
The only reason I know of thatr some folks put Future over the decals is if they’re going to use a weathering technique that works better on a smooth surface (sludge wash for example). If you’re not going to be doing any kind of technique that requires (or at least works better) on a smooth surface, then there’s really no need to apply Future again.