Google:
static electricity shock from belt sander

---- and be prepared to spend an hour or two.

As others have said, an important clue is when it mostly happens on real
cold days, when the RH is quite low.

A point that many seem to miss is that it's the action of the belt rubbing
the rollers or the backing plates that rips electrons off the BACK of the
belt. This leaves the OTHER side of a non-conductive belt deficient,
creating the net charge on the side you are using.

We used to make a simple machine called an "Electrophorus" when we were
kids, that used this phenomenon to good advantage.

One way we bled off static on a particularly sensitive piece of microfilm
equipment was to wipe down everything with a weak solution of Dawn
Dishwashing soap. You might just try spritzing a little water into the air
around the sander.

Pete Stanaitis
---------------