@Stephen B. Thomas thanks for your response! I use a commercial product:
https://www.auson.se/wp-content/uploads/2020/03/Black-Pine-Tar-ENG-200213-1.pdf
Unfortunately when applying the tar, I did it outdoors, and it was moist. I did that in order to prefent excessive fumes. After 4 days I moved the gutters indoors. After another 4 days I turned on the heating of the room, and right now, after almost 2 weeks, it seems the tar is slowly drying! It still feels sticky, but not as fluid as a few days ago. I think I have to be patient?
The only "problem" I have are concerns about the shed having no gutters at the moment, as rain water is spashing against the wooden walls. The wooden wall starts about 15 cm above ground level, as the measured foundation wall extends at least 15 cm above ground level. All wood is painted, so I guess it can handle some splash water for antother 1 or 2 weeks?