On the advice of a club member I finally chose a solution of ARIEL washing
powder which contains bleach and the use of a stiff brush.

At 11:19 31 July 2016, Casey wrote:
I use "Wet and Forget" on my shingle roof, soffits, garage door, and
concre=
te drive. Each time it rains it cleans. I've used Simple Green on
aluminu=
m camper to clean mildew and also on the rubber roof. In early '90's,
the
=
Gov bought Simple Green by the 55 gal drum. Don't use AJAX bleach
alternat=
ive on fiberglass. It will dry it out. It cleans good and maybe ok if
you=
rinse very well then paint. But I would use Simple Green. =20

I don't know why Cobra uses the gelcoat they do. Maybe more flexible
than
=
what boat manufactures use. But I've seen boats out in the sun twice as
lo=
ng as a cobra trailers and looked twice as good. Maybe glider pilots
don't=
wax their trailers. To me, Cobra trailers look more porous than boat
gelc=
oat and oxidize way too quick for what they cost. And I have always
wonder=
ed why an Aluminum cobra trailer top is a different white than the
bottom.
=
I like Cobra trailers, but maybe because there is not much competition.
B=
ut they cost way to much.

I just ran across this that looks promising:
http://www.biokleen.com/hull=
-cleaner

I noticed you said glasspaper instead of sand paper so you are probably
not=
in US where some of these products are available. Since you are going
to
=
paint and have used a wire brush, something harsh like the AjAX bleach
alte=
rnative or something like toilet bowl cleaner or muriatic acid may work
but=
I would rinse very well. People use toilet bowl cleaner, or hull
cleaner,=
or muriatic acid on boats to remove stains.

Good luck.