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Seriously, folks (though I like the "ancient astronauts" theory), blush is indeed often caused by humidity, and is more common in hot weather. It occurs with fast-drying coatings; aircraft dope is notoriously prone to blush. It occurs when the surface is abruptly cooled by the evaporation of the solvents, which causes moisture to condense into droplets. Meanwhile, the coating "skins over" trapping the droplets. A mist coat of the coating or a compatible "slow" solvent is the usual cure. With aircraft dope (on full size aircraft) we add "retarder" to slow the drying time of the coating.
Today's science lecture with practical applications!
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