Dry polishing in a Lot-O

One of my other hobbies is metal detecting, and I bought my Lot-O tumbler partly because I wanted a quicker method of cleaning up grubby looking clad coins and pennies than my old rotary tumblers provide. (Any potentially valuable coins I find are not cleaned in a tumbler -- if at all.) The Lot-O works great for both initial wet cleaning with ceramic pellets and dry polishing with corn cob or walnut shell, however...

When dry polishing in the Lot-O the media becomes loaded with fine rubber grit that's been eroded from the hopper. (Or so it appears. I get the same thing in a rubber rotary drum -- it just takes longer.) After about 3 loads the media needs to be discarded. Do vibes with plastic or poly-vinyl hoppers turn the media black also? If so, is it really material eroded from the hopper, or is it just stuff that's scoured off of the coins? Even without any rubber coming off of the hopper I'd expect the media to eventually blacken -- especially corn cob treated with polish and carnauba wax.

When I say that the media needs to be pitched after 3 loads it's because it starts leaving gooey little black smears on the coins.

So... do the vibes with hard plastic hoppers work better for this?

Thanks,
Mark