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It's probably best to avoid fan motors with brushgear. RC enthusiasts these days use brushless motors with speed controls that could be selected to work off 12V. I dont think the heat in the motor could cause ignition but it wouldnt hurt to check the flash point of the solvents in your paint to ensure that is the case. Come to that, the small fans used in computers are brushless and work off 12V. If you obtained several of them from old PC's they might shift enough air. Another thought is the way a Davy lamp or minor's lamp works. (see Wikipedia). It uses a fine metal mesh that air can pass through but a flame can not.