I am running a new install of Linux Mint 19.1, MATE Desktop (64 bit) and GIMP 2.8.22.
Brand new installs of GIMP 2.8 like to open with a spray paint tool selected. Accidental click of the mouse over the image canvas can damage the image.
I forgot how to change the tool and set about asking for help, then gave that Preferences page another look. So I will answer my own question.
Solution:
Go to Edit > Preferences. In the left panel, select Toolbox.
The tool list will have the default tool highlighted. See the image showing the Toolbox Preference image. It shows the Measurement tool yhighlighted, the Measurement tool is also highlighted in the Toolbox itself.
To change the default tool:
Find the one you want, double-click on its line.
To Save your changes: In the left panel, select Tool Options
This is just above the Toolbox. Click to Save Now or to Save on Exit. See the Tools Options image, where I drew a box around the save options at the top of the Preference window.
-- AZM