Jaba,
Careful, you might give some of the militants in CASA ideas.
Seriously, in this day and age, there are better things than linseed oil to fill tubes after welding. Having had some experience in such things, I would not go for a hard coating, but any one of a number of the coatings that are both a mechanical barrier to moisture, and have anti-corrosion properties.
Tootle pip!!