[morzh]

For the future:

It seems that when you were prying out the tar, you levered the prying tool against the wall and the rim of it (there are dents there).
It does not have to happen. After you are done with most of the core, the sides are thin enough that if you run flat screwdriver between it and the wall, it will just separate without you having to push it against the wall.

This way you will keep it from denting.