Canon BJ-20 User Manual page 84

Cleaning the Print Head
The printer's unique head cleaning system periodically (automatically)
clears away paper particles and accumulated dust that might otherwise
cause printing problems. The cleaning system also clears the nozzles and
helps maintain a high level of print quality. However, if streaking or
printing gaps appear in the output, clean the print head.
For details, see the "Cleaning the Print Head" Section in Chapter 2
Getting Started.
Cleaning the Printer Cabinet
Clean the printer regularly to avoid print problem.
Make sure the printer is off and the AC adapter disconnected. Use a
soft, moistened cloth to remove any ink mist or paper debris from the
inside of the printer.