Hallo Hans,
somehow I have missed this thread so far - though I like old Carette very much!
It's good to see her running again, congratulations.
How far have you proceeded with the restoration? Actually, I wouldn't try to do too much...
Do you refer to the blueing of the firebox or of the boiler? Some of these Carettes came with blank brass boilers, and I am nit sure if that on your engine ever was blued at all...
Regarding the sight-glass seals - I hope that the plasticky stuff comes off easily, usually it does when the boiler is hot...
Re-sealing can be done quite easily using slices of silicone hose, 2-3mm thick..
You can be glad to have the lovely Carette water gauge cover on your engine!
Good Luck
Thomas
PS: Going to post a few pictures of a Carette I recently got running again.