J Barrett Irby Jr
You are correct. The first time I did this I had a fine belt, maybe 120 grit on the sander and it worked well. This time it was something like 60 grit. That may have been the problem. Interestingly, I had cleaned the shop to the point of trying to blow it out with a leaf blower. But, there was an accumulation of dust inside the sander in spite of it being attached to a shop vac.

I had one more chisel to do so I did it without the shop vac thinking there was less chance of sucking a spark into the sander. At that point I had blown out the sander pretty thoroughly so there was no fuel in it.

Interesting idea about the size of the grit/sparks. I should have changed the belt to a finer one just to get a better finish but I was too trifling.