Just passing along my recent experience with cleaning
fenders that worked extremely well for me.
I have a white Taylor 8 x 23 & a white with blue ends Polyform F3.Both were badly marked up & scuffed with black embedded in the scuffs,as well as stained on the smooth areas.
I tried several different cleaners : Detergents/soaps,Spray Nine,Fantastic,GooGone,Laquer thinner,Javex chlorine bleach 5.5% & Acetone -all with a cloth & a scrubbing pad.
Most of these cleaned some areas reasonably well,but none could clean the scuffed areas.
I then borrowed a method used by lobstermen to clean fouling from their bouys.
I took a clean plastic 20L/5Gal pail,added a gal of Javex,(less may work)stood fender
in pail,used a bungie cow hitch to lash top eye of fender
to the bail & filled pail to brim with fresh water
. Let sit for a couple hrs.(may have worked sooner).
Removed fender & rinsed bleach off-beautiful snow white.
Inverted fender & repeated other end.
After rinsing & drying,I waxed fenders with Nufinish car wax to seal surface.