I usually attempt to keep my painting to as few layers as possible as each one covers up a certain amount of detail.
I have heard of the Future fix, especially with canopies, but I haven't tried it yet as a glossy finish.
To determine whether the finish you're spraying is going to be flat or glossy, it depends on the brand of acrylic paint you're using. Count on Pollyscale to be flat. Count on True Color to be glossy. Other brands sometimes specify if their finish is flat, semi-gloss or glossy, instead of just expecting you to magically know.
For myself, I prefer to shoot Scalecoat II (which is glossy), because I prefer enamels. I think the finish is more durable and also enamels are easier to control in my internal mix Paasche VLs and my Paasche AB External Mix brushes because enamels don't cure while inside my brushes, changing the properties of the paint while I'm still spraying.
However, I do like the flat finish paints for weathering or painting structures. I use my external mix Paasche AB turbine driven brush to spray them, so if the paint starts to cure, I don't have to worry about cured acrylic making me have to disassemble my brush and throw it into my ultrasonic cleaner before using it again, or having it build up on the needle and "blob off" onto what I'm painting.
The secret to a flawless glossy finish is to make sure your pressure and distance are just right. Too much pressure, and the paint dries before it hits whatever you're painting causing pebbling and orange-peel. Move your model closer and with too much pressure, the paint starts to run...you don't want that! The right combination of pressure and distance will allow the paint to hit the surface still wet, but not wet enough to run. Then, it'll cover smoothly, and glossy paint will be very glossy indeed.
You just have to practice. I use old junky cars that I would never run on my railroad to practice with, so if I screw it up, there's no loss. Then, I write down my pressure and distance for that brand of paint.
I also always use an inline Paasche moisture trap in addition to the moisture trap that's on my large compressor. Any water that's in the line can really mess up your work and NEVER assume there isn't water in the line.
