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Is it supposed to be removed mechanically or chemically? Or is it supposed to stay forever? I need to redo some solder points for some components but I guess this is not possible any more, right?
This seems to be conformal coating. You can check this page for some information on removal and identification of the coating. https://www.vaniman.com/ultimate-conformal-coating-removal-guide/
Yeah, it's probably burnt beond repair.
You could probably try and clean that up with some contact spray... or scrape it off with a knife... somehow... IDK, a few more pics would help a lot. Is that plastic?
It's some kind of potting material - it's hard to say from the photo but it looks like it might be soft? It's probably been put on to protect the board against moisture and pollution. You may be able to scrape it off with a scalpel blade or similar if you need to. Depending on what the material is it may melt away under soldering heat or it might burn, or it may be quite heat proof