I have a StCroix rod that I built that has a matte grey unfinished surface. It makes a great stealth rod and had almost no rod flash. No problems so far in 4 years of fishing,
'U-40 perma-gloss' is a 1 part extra thin urethane finish made for this application. You could also use a spray on urethane from any hardware store. Urethane will probaly be more durable than other finishes, including most epoxies.
Consider that adding even a thin layer of finish will change the diameter slightly and you might need to re-fit the ferrule connetion. You might want to mask off the male ferrule contact area to prevent problems.
Stay away from spar varnish or other finishes that might become sticky and cause the ferrule joint to bind.