49 C.F.R. § 176.325 – Smoking or open flame and posting of warning signs.

(a) Smoking or the use of open flame is prohibited in any hold or compartment containing a Class 3 (flammable) or combustible liquid, near any Class 3 (flammable) or combustible liquid stowed on deck, or near any ventilator leading to a hold containing such material.
(b) A sign carrying the legend:
FLAMMABLE VAPORS KEEP LIGHTS AND FIRE AWAY NO SMOKING
must be conspicuously posted at each approach to a Class 3 (flammable) or combustible liquid stowed “on deck” and near each cargo hold ventilator leading to a hold or compartment containing this material. This sign must be painted on a white background using red letters. The letters may not be less than 8 cm (3 inches) high.