Thanks a lot for your responses guys! I'll take them uder consideration.

The bike is an old mercier and it's quite beautiful.

I purchased and use a wire brush and I must say that's such a great tool for rust scraping or generally steel polishing. Way better than sand paper if you ask me.

Also I think the paint work always will be visible if you try to cover just some spots and not paint everything from scratch because time wore away the old paint and made it very difficult to duplicate.