Thanks gentlemen for the kind words on these planes. It's definitely been a fun adventure and I still can't believe how much I have learned about the Spitfires during this trip.
Pedro I use Tamiya X-21 from a bottle.
It's called "Flat Base" and is for use with Acrylics only.
It has to be mixed up very well or you will get a bunch of tiny little white specs all over the finish and that will destroy all of your hard work.
You can not use this stuff straight from the bottle. If you do your headed for a disaster.
What I do is I mix it with Future clear acrylic floor sealer. I use about 70 percent Future and 30 percent Tamiya X-21. You can experiment with the ratio but this works well for me.
Now that you have mixed up the X-21 into the Future, it needs to be thinned out for spraying.
I use it thinned out with regular medical use rubbing alcohol.
I mix it up at a 50 /50 ratio. You can add more alcohol and it will dry faster and spray easier. Do some experimenting and see what works best for you.
I spray it on the model in numerous thin layers. I allow it to flash dry between coats. It dried very fast. You can even "blow dry" it using your air brush to simply blow air onto the surfaces. You will see it dry right before your eyes.
If you want it to look more flat instead of shiny, keep adding more layers and before you know it, you will get the look you're after.
If you're going for a more "flat" looking finish then you can add a little more of the Tamiya X-21 into the future. I would really add too much because it seems to work better with less.
This is something that you can experiment with to see what you like best.
One minor drawback is that on occasion it can give your model a slightly off white look at certain angles. But it's really not very noticeable and I think that it gives your model a more scale effect that lightens up the colors just a little.
Hope this helps ...