If you have an aluminum fan Smooth it out for performance.

[Genbiltstein]

I discovered several issues when dealing with the 450sl.
I have noticed that the radiator fan built up a large amount of heavy scale over the years.
At first I thought that Mercedes Benz and company cast that fan with a very rough surface. I am sure when that car left the factory the fan was smooth.
Heavy scale on an aluminum fan will degrade the performance because of the turbulance delevoped. Smooth that sucker out.

Inspecting: Run your hands on the inside and the outside of the blades of the fan. If it is rough then they need to be polished down. The smoother the better.
It is a mystery why scale builds up in the first place. Time + friction + Static electricity = corrosion is my guess.
Remove the fan and get a couple of sheets of emery cloth. "Sand down" and really polish the rough spots. Pay attention to the inside of the fan blades.