Air brush your top coat like u r air brushing other colors. Afterall, top coat is basically a paint without color.

Many has this idea that top coating is after u assembled the whole kit, then "Psst...." the whole kit a few times with top coat. Well, if u do that, i would expect next thing u will start to ask question like "Why got frosting?" "Why not top coat even" "why not enough gloss?" etc etc

Again, top coating is spraying another layer of paint that has no color onto the kit. Spray part by part.

Be careful, Tamiya flat base, or other brands' flat base, is just an addictive to be added into clear.

Ok the concept of top coat:

paint consist of the resin n the pigment. paint with red pigment is red paint. paint with pigment, just only the resin, usually what u ll see in the market is usually labeled as "Clear" or "Gloss". This type of resin, we spray on top of our paint kit surface, to serve as a protective layer onto the paint. Since it has no pigment, it will not alter the color at the bottom. (Same concept as u use a transparent plastic sheet to wrap your book during school days).

In short, those spray can "top coat" is basically same thing as those bottled "clear" or "Gloss".

The resin, however, when sprayed and dried, it tends to level up itself and as a result it looks kinda shinny or glossy. So we can add some additive into it: some micro-size powder, to make the final outlook to look more flat, or less gloss. The more u add, the more flat it will be. This additive, micro size particle is generally called "Flat base". So basically if u look at Gaianote's or Mr Hobby's Semi gloss or Flat, u can achieve the same effect by getting a bottle of gloss, and add flat base into it. Add a little, it will be more like semi gloss; add more, it will become flat.

This additive micro particle, basically is just particle alone. So if u spray it without adding it into the resin (clear or any color), where there is nothing to hold them together; when it dries, it will become like powder on top of your kit: Like u have sprinkle some baby talcum onto the kit.

So flat base CANNOT be used alone, it must be added into clear or color (Eg adding flat base in Yellow u will get a semi gloss yellow, or a flat yellow).

So i guess what u should be looking for is color labeled as "flat", or if u r using tamiya acrylic, u should get yourself a clear n flat base and mix them urself. If i m not mistaken, Mr Hobby n Gaia do manufacture a pre-mix flat, but tamiya doesn't.