Scanner Glass - Dirty or Damaged?

I bought a used (I know) Canoscan 8600F. Overall it's in good condition, and I've gotten some decent film scans from it. But for the life of me, I can't seem to get the glass clean. Without the lamp, it always looks crystal clear -- like I could reach my hand into it. But viewed at an angle with the lamp on, I see dark (maybe even patterened) streaks that don't seem to wipe away. Could it be that the glass is damaged? Are the streaks on the other side of the glass? Yesterday, I scanned a paper negative (very dark) and the streaks show up in the scan.

Is there a trick to cleaning scanner glass, or did I get a bad deal? It's really frustrating.

Thanks.

my guess: you have to open the scanner to clean the glass from the rear side. Search google for 'service manual canoscan 8600f' or 'open canoscan 8600f'.