Tim, we've had several instances of fires caused by self-heating of rags soaked with wood stain or polyurethane. These oil-based products require several days to cure. The curing process is exothermic. If rags, rollers, etc.. are not handled properly ignition by self-heating is possible. Rags should be laid flat on a noncombustible floor or individually hung in a well-ventilated area to dry. Even when they are dry to the touch, the curing process may not be complete. It's advisable for them to be left for a few days after they have dried. Also had a couple instances in which the sanding dust (I believe it's referred to as excelsior) was left in a plastic bag or a densely packed pile and subsequently self-heated to ignition.