Some sort of clear finish is often applied to blued or browned Damascus barrels to add gloss to the final finish, and to protect them from oxidation and corrosion. It maintains the contrast better than allowing them to age naturally and unprotected. Some of these clear finishes are better than others at doing this job. Boiled Linseed Oil is often used, and BLO is a very poor moisture barrier compared to say, clear lacquer. Things like Tru-Oil and varnish are also used, but while they reduce transmission of oxygen and moisture, they are not impervious.
Any clear-coat can eventually fail or wear thin, so they may need to be removed and replaced at some point to continue to preserve and protect the finish. It is necessary to strip the old clear coat for best results, but some guys might simply spray or brush on another coat and call it a day.
I personally like to use clear lacquer for this purpose because it goes on easy, remains clear, and dries fast. But I have been wanting to try spraying a 2 part catalyzed automotive urethane clear coat like the DuPont 7900S I've used on vehicles.
