Well you should know the one i mean - gets stained yellow-brown over years

and is difficult and unsavoury to clean back to gleaming white.

Well my 2-bobs worth -

(1). 

Clean it up with Jiff or the like then spray with VANISH Oxy-Action and leave it

for hours. Maybe overnight. the spray is thick so hangs to the vertical surfaces

reasonably well.

  The best one to get is the GOLD spray which is 50% Hydrogen Peroxide and is

consequently more effective than the 30%. It is harmless to plastics so no worry.

  When it is clean and white - which is a project for when in lay-up and likely after

a number of applications, Polish the bowl with a good marine quality wax polish.

(2) When the hole is nice and clean, and especially when new, a spray after a

wipe with metho or better still Acetone, with a clear lacquer from an aerosol can.

Yep, I sell Vanish. 

B