Tamiya Clears

Stay away from using X-21. They're nothing but garbage and not worth the trouble using them unless you're looking to ruin a perfectly nice kit. Not worth adding it to gloss paint when there are flat paints available; hence the letters XF before the number.

XF-86 is a flat clearcoat. 

Tamiya's X-21 Flat Base is used to tone down gloss paints. A good discussion of its merits and flaws can be found here:

http://cs.finescale.com/fsm/tools_techniques_and_reference_materials/f/18/t/58831.aspx

XF-22 is RLM Grey is an opaque paint that has plenty of pigment. Not sure how it fits here.

XF-86 is a flat clear for a final coat to seal the model. No color, but the chemistry of the sealer causes it to dry to a non-reflective finish. XF-35 is also colorless, but dries to a semi-gloss finish which is slightly smoother and more reflective than flat. The X-22 is the gloss version, which makes things very smooth, shiny, and reflective. 

You can mix Tamiya's paints to tint the clears to achieve different layers with translucency, but that requires a bit of experimenting to keep a balance of paint, clear, and thinner to get it right. You'll see a lot of old school armor modellers using Tamiya's buff with XF-86 to get dust effects and such. It can be an effective technique.

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KnightTemplar,

Thank you.  Your overview is very helpful.

Pat

I read somewhere that Tamiya clear gloss acrylic thinned with Mr. Color Leveling Thinner will lay down a glass smooth gloss shine. I have to try that one day.