Dirt is in the air, dirt is everywhere. Especially in humid environment like clouds.
It is not like dust, actually it is rather greasy. Areas being very dirty: everything behind the engines (especially with the DC9's and 727's and all other acft with rear mounted engines) you could see the oily-greasy dirt very good, followed by the window areas, doors (pax+cargo). Most difficult cleaning is the nose/cockpit area plus the front sides of the wings, engine intakes - because of the insects sticking like hell to these parts.
The problem with cleaning aircraft is - it is mostly done manually. LH
started in 1996 a fully automated "aircraft washing" but this was not succesful - even with the best programmed robots there where damages to antennas and so on. A pic of the "wash-robot Skywash" can be seen here: http://www.dynajet.de/showpage.php?akleft=1&subakleft=18&site=16&anident=49
Skywash covered almost 85% of the aircrafts surface, remaining 15% had still to be cleaned by hand.
Washing is an integral component for reducing fuel consumption. A "normal-dirty" 747 needs about 1% more fuel over a clean one. And this sums up.
Normally an aircraft should be washed after 45-60 days of operation. In case of areas closer to the sea or flying in dusty environments (volcanic areas) this can go down to 35-40 days. Usually washing is done with maintenance cycles.
Full wash of a 747 requires around 25.000 liters of water plus cleaning material and many many people....
Somewhere in Japan they are working now again with a "robotwash" since a couple of months. Robot-Washing takes about 2-3 hours for about 80% of the aircraft, remainder done again manually.
When you google a little bit with the words "aircraft cleaning" you do get some companies listed and some have pictures showing (small) aircraft cleaned manually.