I do it both ways, brush on some models (or spots), and airbrush on others
I don't have to thin it either way, one tip about brushing it on is to not use too large a brush, or if you do use a 1/4" flat, unload it some so you don't get a flood of Future
some people also provide a means of letting any excess get off of the model, to avoid pooling,,,,,,,,but, if you put it on thin enough, you won't have that happen
also, you don't need a "very high gloss" finish to apply decals, one SG coat of Future is enough,,,,,,,,,decals are designed thin enough to go over 'some' highs and lows, so a slightly not perfect gloss is enough
if you get a "bad spot" when your Future is dry or nearly dry, don't worry,,,,,,,,,after you let it dry for a full day, you can sand a bit on the offending spot and then reapply Future to that place,,,,,it will blend in just like the "perfect" coat you laid down on the rest of your model