IF you want a smooth surface and high gloss finish, you should not use x-22, its acrilyc not lacquer, and... its hard to get a smoth surface and glossy at the same time, and further, you wouldnt be able to polish it, this paint is really... how is it said...well, not strong, and with the sandpaper you'll run out of clear and you'll sand the paint.
If you still want to use it, you can clean the airbursh with alcohol, pharmacy one, and use and solvent isopropyl alcohol, its much cheaper than tamiya's specific one
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If I were you I'd use what kasso asks if its a clear coat or not, tamiya clear spray can, yes this is the clear coat that you should use, I use duplicolor, its cheapest and comes with more paint, but tamiya is supposed to give a better gloss, but like I still dont get high gloss without a deep polish... its the same
but someday I'll get highgloss directly from can!!