Noticed that on my Hoover 825 that someone had used it as a shop vac at one time, and there are impact divots on the discharge of the housing. The impeller/fan/rotor doesn't come close to it, so I'm going to experiment and paint some rubberized coating like you use on tool handles on the interior of the housing since it might pickup the occasional screw in my house. Results will be released as the study goes on.
Actually, my own post got me wondering. I've seen "specials" that have a metal ring retrofitted around the fan housing for protection. Do any of you guys have one, or a pic of one?