If you want to increase the diameter of your pellet, it might be better to push it through some kind of die that very slightly compresses it.

The answer would really be to find another pellet which fits the bore better, or toss the gun and get one with a proper barrel.

Coating pellets with gunk is retrograde and the kind of thing people did when they only had black-powder muskets, irregular balls, tow, paper, wobbly ramrods and too much time on their hands.