Fly Line Treatments

Fly line coatings should be slick to reduce friction when the line is travelling through the rod guides improving casting distance. Over time when lines are being used they pick up dirt and grit and lose their slick coating. Fly line treatments and cleaners are used to clean the line and help restore the line's original slick finish.
Leader treatments are commonly used to degrease leaders and tippets to encourage them to sink. This is particularly useful when fishing dry flies as a leader floating on the surface is obvious to fish looking from below.