Got back from Hamble and struggled to remove the strange brown stains from Gogo's hull. Keith suggested I use tar remover, so tried that; vinegar, starbrite cleaner and alloy wheel cleaner. The latter worked, but has other chemicals in it apart from phosphoric acid....

Then picked up a post from the Hamble Boating Group on Facebook, where a commercial boat training owner suggested Grunt and showed the result on their ribs.

Brushed on, washed off with a sponge an water 30mins later, the product worked very well and all gone. A word of warning tho' - protect your trailer from splashes, as it fancies the galvanising...It's <5% phosphoric acid, but is still very effective.