Erase Tool allows to erase the picture.

Hotkey is CTRL + SHIFT + D.

It has different sizes of erase cursor.

(!) It is important that you can erase with varying times of pressing. If you press LMB one more time it will erase more deeper.
Also, with 100% opacity erasing is full, with less opacity level erasing is not full as you can see below.

Here you can see different opacity levels using Erase Tool. You will get these opacity levels, erasing n number of times on the same path (from top to bottom equal 1 to 5 erasing).