atmospheric humidity is one source for moisture I the finish, the compressor is another if it is not correctly used. Compressors collect moisture from the air, and can put it right back into the finish. Chuck mentioned the inline gun filters, those are a great, inexpensive, and are should be a backup, last line of defense from contamination in air lines. Shop air plumbing is very important. Coming off the compressor into a vertical stand pipe, 3/4" copper works well for this, into a 'T' fitting, with a 1/4 turn drain on the lower leg to periodically release trapped moisture. The theory of the metal vertical standpipe is that the (hopefully) cooler temperature of the vertical metal pipe will encourage condensation as it enters the pipe. The top of the stand pipe steps up to 2" galvanized pipe sloped at a gentle angle on the wall near the ceiling. Air going into the lower angled pipe slows down, and moisture condenses even more, and since it is sloped towards the vertical stand pipe, condensed moisture drains back into the vertical pipe. As long a run of this as possible is good, or at least 15-20 ft. Then another vertical 3/4" copper drop, provides air to workstations where you want air, with air feeds out of 'T' fittings, and 1/4 turn valves at the bottom to release trapped moisture. At critical feeds, such as for air for spray guns, an air filter like this:
http://www.northerntool.com/shop/to...i_sku=406720&gclid=CLyB3cmj7rgCFbCDQgodKWwA4Q
adds the next line of defense from contamination. Being sure to never use spray air hoses to deliver air from unfiltered, 'dirty' sources ( straight out of your compressor) is important, and using inline gun filters. In my experience, the vertical pipe right off the compressor traps a lot of moisture.
If you take none of these precautions, just using an air line straight off the compressor, you can almost be assured of moisture in your air at some point or another.
I have not had blush problems from spraying unthinned lacquer with Cardinal, or other lacquers (only did that at first to 'learn' Cardinal in my environment/ tools, I always thin it now) I do not need to use retarder either. I do not live in what would be considered a humid environment. It is possible your gun/ spraying technique/ settings would work without thinning, but thinning will almost certainly give you a better finish. Unthinned Cardinal could cause you a lot of headache trying to get it 'flat'.
It is easy to get thinner to spray 'what looks correct', you need to set the gun for the exact lacquer mix you are spraying, and most likely check it every time unless you are doing a lot of production and just pumping finish through the gun. You cannot really see much of value by looking at how thinner sprays, maybe just to check if there are major clogs in various places in the gun, which there should be none.