Corners of the new windows are very dirty or scratched

Absolutely not acceptable at all!

So, we have now agreed. I will (try) to sand down and clean the corners as best as possible. As a goodwill gesture, we will get a discount from the window fitter on the interior window sills.

Because the interior window sills cost so much. Just don’t let yourself get involved in such nonsense!

Google tries, before such nonsense is written here?!

There is a "Guideline for the Assessment of the Visual Quality of Glass in Construction".

It states that certain "defects" are not defects.

The directive has no legal relevance whatsoever.

Trying to use Google and being able to assess the text found are two completely different things! It talks about glass and not about (plastic) parts of the frame. In addition, the guideline is described as "application guideline within the relationship between manufacturer and processor of glass products." (Source HP glaswelt). This means the customer is then also excluded. Besides, it is only a guideline and not a standard.

By now, I am always a bit ambivalent about such things. If they screwed up, they should fix it. However, I also tend to do things myself sometimes so that it does not get worse by botched repairs. If it is going to be good, do it yourself...

Stupid situation, but to some extent, I can understand the self-fixing. However, I would not accept the "trick" with the cheaper window sills. There should be more result than that.