(06-13-2020, 12:19 PM)Gerard Wrote: I have tried changing it to 100.0 and quitting Gimp - but when I reload and try again the default is back to 0.0
Try setting the opacity to 100 then in Edit -> Preferences -> Tool Options Click on Save tool options now and in Input Devices click on Save Input devices now OK the the preferences dialog and restart Gimp. See if it makes any difference.
Some screenshots from another similar post https://i.imgur.com/hNZOQ0O.jpg
Other than that you might have to reset to default values