To protect your investment in expensive offshore trolling reels with aluminium spools, always grease the entire surface of the spool with marine grade waterproof grease before putting the line on. This will prevent or minimise corrosion resulting from saltwater on the line. This can be  especially problematic when a fish takes most of the line out during the fight - when the line comes back on the spool the salt will be deep down close to the metal and will not be removed when washing the reel at the end of the day. Re-grease the spool every time you change the line to avoid this problem.