Industrial ceiling fans are hardworking pieces of equipment, usually all steel, that will acquire rust spots over years of use. Maintaining and restoring industrial ceiling fans requires a bit of work and is a messy job, but it's well worth the effort.
Step 1 - Inspect and Clean the Fan
If the fan is still installed on the ceiling, first turn off all power to the fan. Use the ladder to remove it and bring it down to ground level. Inspect the fan to determine where all the rust is located. Wipe off as much dirt and grime as possible with old rags and discard the rags when you're done. You will probably want to wear work gloves during that entire process as the fan is likely very dirty.
Step 2 - Remove Rust
Put on your safety glasses. Using the drill with the metal sanding attachment, grind away as much of the rust as you can. Keep the drill moving so as to not cut grooves into the metal. Continue on all rusted areas of the fan.
After you have removed as much rust as possible with the drill, switch to steel wool. You may dip the steel wool into mineral spirits to aid in the rust removal. Scour away any fine rust with the steel wool and clean with the mineral spirits. This should take your fan down to bare, clean metal.