I use some magic gunge called Pro Marine Hull Magic, I think there is some similar stuff called Y10. On the pot it says 'the ultimate GRP stain remover'.It gets off all stains I've had, that nothing else will touch.
I get it from my local chandler in Poole. I'm sure it must be stocked by others. Inevitably, as it's for boats, it's not cheap. £11 for 200ml pot. Goes quite a long way.
And as many others have posted here, Oxalyc Acid from a chemist and mix with water and wallpaper paste (to suspend). Very cheap and is what you pay boaty prices for in boaty bottles.
Is that what it is? £11 another right rip off then! Next time I'll use your recipe, mind you, it is a nice pink colour, so need some harmless colouring as well!
Hmmmm! Geoff, I thought you were aboard my boat when Mike Haley showed me how good DETTOX was and that stuff is only about 85p or so a spray bottle. What amused me afterwards was when it mentioned it to my Gruppenfuhrer. She knew about it all the time and produced a bottle from under the kitchen sink. Proving maybe we should always consult our wives first on these matters.
Possibly you need to be thinking of cause as well.
Try a .22 for the birds and a bushmans for the plant then
Flash and elbow grease but which part of this sceptre'd isle do you inhabit to still be getting blackberries - ours went long ago