If you have an airbrush, it's easily sprayed. Future is really nothing more than a clear acrylic paint, even if some people think its for floors. Like all paints it runs when laid on too thick. If you managed to lay down a nice coat using auto paints, you'll have no trouble.
It can be brushed, but use a CLEAN wide brush. Future has excellent self levelling properties. Much better than pigmented paint. I suspect that has to do with the lack of pigments in Future.
If you are more comfortable using a "regular" clear coat, by all means use them. Let the Future cure for a minimum of 24 hours after fixing the decal. This will also let you know if it worked or not.