How often do aircraft get an external clean?

[cee cee]

Originally Posted by PDR1

The point is that the spoilers are not raise FOR landing; they are raised AFTER landing (they cannot be raised unless the aircraft has weight on its wheels.

Actually they can be deployed while the aircraft is still flying. I have seen them deployed for short periods when descending on approach to the airport. Wikipedia says

Airliners are almost always fitted with spoilers. Spoilers are used to increase descent rate without increasing speed.

It is true that they are only deployed fully and for a sustained period after touchdown.