Sticky residue on doors from the 19th century?

Hello,

while I am waiting for approval from the [Denkmalschutz] for facade work, I wanted to do something useful and fix some interior doors. These are covered with a dark brown, sticky coating. I tried to dissolve it with alcohol, like shellac, but that was probably not it. I know that this part of the building was renovated in the 19th century, with walls (and probably also doors) dating from the 16th century. Do you have any idea which solvent I should try?

Best regards

Konrad

Maybe simple cooking oil helps? I always get my [Dunstabzugshaube] sparkling clean with it.

Coating, in the sense of patina? Or paint? In the latter case, a hot air gun can often be very useful. Otherwise, vinegar repeatedly proves to be a miracle weapon in our household.

Are only the doors from the nineteenth century, or also your phone? - otherwise just take some pictures!