I haven't heard of propellant doing this (unless you hold the can upside down
) but I haven't used them in years. Moisture in your airlines can wreak havoc on a paint job, not to mention your sanity. They're only a couple-o-bucks and well worth the investment. I don't think that the lack of one would cause an all-over dulling, that sounds more like painting in high humidity, low temp conditions but installing one would narrow down the suspects and prevent future (ha ha) catastrophies. Hold a part over a disposable cup and pour some Klear on it (indoors) and see if it fogs up. If it does, the probs are not airbrush related but are more likely enviormental or a bad bottle of Klear.
-fish