Tried the toaster with a long metal pin through the center of the dial as You can push it in and out - but it would be better to have several dials around for this kind of "test":) Water based varnish mixed with brownish/reddish colour works well but is permanent. Aquarel may be washed out again.
For the lume dots my fav. results are with acrylic paint mixed with raw sienna or yellow ochre (or Nescafé) and lume powder. When still half wet or totally dry, with care, the dots can be popped out with a stanley knife and be re-done - or filed off a bit with fine grit sandpaper, it all just requires time.
not too period correct but I just needed lume in the dark, don´t like to use them otherwise