Do airlines ever clean their planes?
In addition to repaints, aircraft are also periodically washed and treated to maintain the paint quality and reduce the build up of dust/grime/salt and other particulates.

There are automated systems installed at some airports to clean airplanes as they taxi over these areas. Think of it as a drive through car wash, but for airplanes.

The very first applications of these were actually for the US Navy because they have a requirement to wash aircraft due to their proximity to seawater. A company called tammermatic developed a system for the navy to clean airplanes directly on the field using a set of powerful jets. Images from the tammermatic website





Then there is the Skywash system (PDF brochure), developed by PMW (a company that makes concrete pumps) to wash aircraft automatically. It was first used by Lufthansa in 1997 (image cc: ARNE DEDERT / AFP):