Silicone spray. You can apply directly to the boltrope but I have found it is better and less wasteful to put a slug in an old sock, tie both ends of the main halyard to the sock (so you can pull up and back down) and insert the soaked-with-silicone-spray sock into the channel and hoist up/down/up/down spreading the silicone on the inside of the channel.
Good luck,
Jerry