Steve, I believe that not all the rust is disolved at the same time. Maybe the bond is disolved and hunks float off and sit in the bottom of the tub. Now, the stuff keeps working on that sediment and shortens it's life. I put one of those cotton face respirators in a funnel and poured the stuff through to filter out the sediment. I think that makes it last longer. Also, when you're done filtering, the face mask can be rinsed out with water to be used again.
Verne