Some pictures here of before and after
http://www.hobbyhovel.com/2010/08/tips-and-tricks-spraying-flat-top-coat-on-unpainted-models.html
I used Mr. Top Coat (Flat/Matt) before but it turns out semi-gloss not matt {eek3} . Anyway I am not a modeller ^&grin
The three big choices are the Mr. Top Coat (Flat/Matt), Mr. Super Clear (Flat), and the Tamiya Modeller TS-80.
How to Spray
The short answer would be “push down on the spray nozzle”, but you knew that already, didn’t you? So here’s the long version:
Dip your spray can in hot water for a few minutes. I forget where I got this tip from, nor do I remember why this is helpful, but I think this makes the spray mist a little finer due to the heat’s ability to make the particles in the can more excited (did I get that right? It’s been a while since my elementary science classes).
Hold your models around 20-30cm away and flick your wrists back and forth while spraying. This is not precision work, so go nuts. If you spray the parts too closely, you risk the chance of producing a milky fog on the surface of your model.
Spray continuously for a few seconds at a time, not in short bursts. Shake the can well before each spray to get even results.
You don’t have to spray your model piece by piece, but it is a good idea to at least separate the model into several smaller components (e.g. arms, legs, torso, backpack [if large enough]) to avoid excessive obstructions.
Let your parts dry before spraying another coat. You don’t want your parts to be dripping. How much you want to spray is completely up to you – just make sure the parts are evenly coated.
Make sure to set down the freshly-sprayed parts in a safe manner (i.e. don’t set down a wet part flat against another surface). The coat dries within minutes, so it would be reasonable just to hold onto the part in your hand for a while to have it dry a little.
If you are planning to use decals on your model, make sure you apply them before the applying flat top coat. Decals like smooth and even surfaces, and the rough finish that flat top coat leaves would cause major if you need to apply decals on top of that.