In article , Derek Moody
writes

Depends what you're trying to do. Sometimes a nominally sinking line will
pick up so much gunk (especially in hot weather) that it floats when casting
short - a sort of self greasing effect. Then a degrease and clean restores
it. If you're fishing a floating line and want to get the leader down then,
yes, just degrease the leader.

Agreed, but I wouldn't put Mucilin sinkant on my lines - only the
leader.

With modern lines you don't know what harm the dressing would do to the
material.
--
Bill Grey