I must say I learned to apply decals ling before this site exited. 
I use the same technique but never thought of it as that revolutionary. I spray the model with flat clear, then only apply Future with a brush to where the decals go. Then I spot spray flat clear over that again. My reason is simple: I want to limit the number of layers on the model’s finish. Also, no need to spray the whole thing with a gloss coat as that just wastes product and my time. Not to mention it’s another opportunity to get a run. Yeah, they do happen occasionally…