Fully overcast is absolutely diffuse. What does the facade look like?
That's right – the dim light of a bedside lamp is not meant, but merely a cloudiness that does not let the sun's rays hit straight like arrows.
except for the base area I can't see anything anymore
I feel the same. On the other hand, it doesn't require any special eagle eye and suspicion or floodlight; normal sunlight is enough to recognize the defect. I consider the defect to be clearly limited to the appearance, i.e., no technical defects as consequences are to be expected – however, as a visual defect strong enough that you could not sell me the house on sunny days.
My conclusion is therefore: a price reduction in the order of a comparison of about half the costs for renewing the topcoat is appropriate. And in other words: therein I also see the standard to which expert costs must be proportionate.