The cause of the dirty printouts is now clearly revealed. The dark marks visible in the picture create the rough surface that prevents the wiper blade from cleaning the belt.

Isopropyl alcohol softens and removes most of this plastic, aided by a little scraping of the most resistant parts.

The second picture shows the cleaned roller with a smooth surface that enables the wiper blade to work correctly.

After re-assembly the quality problems were corrected on both the belts that were repaired in this way.