Can't comment on your Aztek, but I do endorse Tom's comments - the Tamiya thinner is really good. Also, cleanup on my Paasche double action is really easy, so I suspect that there is a retarder in there as well. As to ratios, I saw 25% Thinner mentioned - tried it - it works ... so I haven't tried anything else.
Welcome in to the world of the eternally paint-spattered
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Just noticed your last post. Tamiya make a good flesh, gun metal, bronze, etc ... Although not an accurate representation, take a look at http://www.modelsforsale.com/catalog/tamiyapaintcharts.php for their paint chart (just one of many available on the web)