hi there
what type of cladding is it.
is it the metal powder coated, painted cladding on an industrial unit.
because whet you find is that it is an oxidation on the surface due to sunlight over a preiod of time, however there are some very good chemicals that can be applied to restore the cladding panels, as that is what you are in effect doing as opposed to cleaning the"watermarks" off.
that may help
regards
martin