On Monday, August 8, 2016 at 8:14:12 PM UTC-4, Casey wrote:
On Monday, August 1, 2016 at 4:31:01 PM UTC-4, Colin Roney wrote:
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.

Thats probably fine for the algae and using plenty of water to rinse. I would use a self etching primer prior to painting. Then any automotive paint.

1975 trailer: https://www.dropbox.com/sh/9icgfg36j...qIC_8cAVa?dl=0

Self etching primer is used for metal but I doubt it would have any useful benefit on a composite trailer.
UH