I agree with Andy. the Future can be applied by brush or airbrush, with the former giving a better 'gloss' per coat. However, be aware that this stuff is very thin, and will find its own level and pool and run easily, so needs laying off properly. Any drips, pools etc can be absorbed with the edge of a piece of tissue.
To ensure an evenly-toned finish, the entire model should receive a coat, before and after decals. The finishing coat can be done using Future mixed with Tamiya Flat Base, to achieve anywhere from gloss (no Flat Base), through semi-gloss, semi-matt to full matt, depending on the amount of Flat Base added. (Experiment and test to find desired result.)
If air-brushing, ensure that the airbrush, and especially the jet nozzle and needle, are thoroughly cleaned immediately after use, using an ammonia-based cleaner (I believe some window cleaning products work) or iso-alcohol, and then flush through with plenty of water.
You may find that the tip/jet may need cleaning, during use, to prevent a build-up of the waxy deposit which, if left, will dry very hard and glue everything solid! Use a paint brush dipped in water to do this as you work, if required.