A number of years back I serviced all the seacocks on "my" vessel. As I recall, 18 both above and BWL. Most just needed a solvent cleaning and re-grease. The two main engine seacocks most probably had NEVER been out since installation since there was a stringer in the way that did not allow barrel removal. A 3" hole saw took care of that. I lapped those two with valve compound as described above, greased and re-installed. Now, I used wheel bearing grease and I found out afterwords that this was not recommended due to a component of the grease being incompatible with the bronze. All work just fine and I no longer need to use the "torque multiplier" piece of pipe the previous owner needed to cycle the engine seacocks.
As to Marelon...NO experience what-so-ever.