Ok I'll try again. The basics are clean the joint both ends with meths/spirit and very lightly rub a paraffin wax candle over the M part ( some candles today are vegetable oil based, paraffin wax is the one you want ) then when you put the rod together offset the tip guides and gently twist into line as you pull the 2 sections together. The plug in the M section is just to stop the ingress of dirt and water, do not fill it with any kind of epoxy. It plays no part in the strength of the rod, thats the blanks job. Yes you can replace the plug with the correct material. Some people throw their tip section simply because they are not really as good a caster as they would like, not saying thats you. As far as a worn joint is concerned, if I had the rod in front of me I could tell you in 5 mins how to fix it. A tapered joint can just need a tiny amount of material taking off the joint but if you've never done it best left to a rod builder/ repairer. Any fly shop worth their salt will have a staff member who can give you the correct advice.