drumsfield
I think I found the answer to my own question...
http://www.militarymodelling.com/forums/postings.asp?th=42979
"You did not do anything wrong, in fact you had a lucky escape. As Si said Tamiya Clear is added to matt paint to give a gloss finish and Flat Base is added to gloss paint to give a matt finish. If either Clear or Flat Base are sprayed on a model they will ruin it. "
This is incorrect. While X-21 Flat Base is an additive to make gloss colours flat, X-22 Clear is a gloss finish in its own right.
Graininess with Tamiya gloss colors (and clears) is often the result of the paint not being wet enough for it to level. This often happens at the edge of the spray pattern, resulting in each pass leaving a grainy edge which is not covered by successive passes.
Thin X-22 with X-20A acrylic thinner, at about two parts thinner to one part X-22. Airbrush at a lower pressure from a closer distance.
Apply multiple thin coats and do not use a substitute thinner like Isopropyl alcohol as this flashes off faster than X-20A thinner. Though it will thin Tamiya paint, never use water as a thinner.
If you are of a mind to try it, Tamiya lacquer thinner will deliver even better results than X-20A, as it extends the drying time a little more and gives the paint more time to level.