Oily Paint Rags Can Spontaneously Combust
Spontaneous combustion occurs when rag or cloth is slowly heated to its ignition point through oxidation. A substance will begin to release heat as it oxidizes. If this heat has no way to escape, like in a pile, the temperature will rise to a level high enough to ignite the oil and ignite the rag or cloth.
Some oil-based finishes like stains, polishes and varnishes have a tendency to spontaneously heat as they dry and cure.
If rags or cloths wet with these finishes are mishandled, the spontaneous heating can accelerate and might lead to ignition and a fire. How fast the overheating proceeds depends on a number of factors, including ambient temperature, how tightly packed the rags are, the nature of the oil, and other factors.
The Right Way to Dispose of Paint Rags
Do not pile or ball rags soaked with oil paint into a tight mass or toss them in the regular trash while they’re still wet.
Do allow the rags to dry thoroughly before disposal. Spread the rags outdoors, on the ground or on a metal rack, until completely dry and somewhat hard. Two full days is usually enough time, but it might take longer.
Once the rags are completely dry, they should be safe for disposal. Put them in the trash on collection day. This method allows the oil to fully cure without overheating.
If paint rags catch fire outside, extinguish the fire by dousing with water or covering with sand or dirt. Don’t disturb them until you’re sure the fire is out.
If paint rags catch fire indoors, call the fire department, get everyone outside, then (and only then) try to put the fire out with an extinguisher. If you cannot put out the fire, get out of the house and wait for the fire department.
Don’t try to put out the fire with an extinguisher unless you know how to operate it.