After using a mild degreaser I started removing the original paint. This time I opted for a set of brushes displaying different bristles (steel, cooper and the third one nylon).
First used a paint remover and let it work for some minutes, then attacked the softened surface with the steel brush trying not to scratch too much the tube's surface and finishing off the job with the more gentle cooper and nylon brushes.
The last step is a throrough water-sanding to level off the small imperfections.
That's how it looks like.
Brass brazing, isn't it? There's an ungainly blob clearly visible We'll talk about it later.