yep, it might sound daft, but "water" is the very thing you need. I'm not the only one who's tried it either, and i think a few other forumites will confirm that "water" is pretty much guaranteed to be ok and not harm the grp - but as always TRY A SMALL AREA FIRST. Now, i recognise that so far you have only ever tried a small area, so this time, having tried the small area you can continue to a slightly larger area. Don't worry if at the end of the cleaning you lose all that ghostly gloomy grey colour of the boat and it ends up white.
You need to send a post to hlb for some more of his "boat remover" chemical cos altho the stuff i gave you was in a vodka bottle and that it was quite funny for you to be clanking along clutching a bottle of vodka, it wasn't vodka inside it...
i needed to get back ter the car! It's honestly not v expensive anyway. I mean it could be anyway could cos i mean if a yorkshireman gives summink free to a scot...