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My outside wall has a stain on it that I can't seem to remove. It's a roughcast wall and the stain was caused by the boiler overflow outlet.

I have managed to lighten the stain slightly with different cleaning products and careful treatment with a pressure washer, but it won't come off completely.

Are there any other things that I can try?

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  • I do not see a Boiler over flow? I see what appears to be an Iron loop. Rust.
    – Alaska Man
    Apr 9 at 19:25
  • @AlaskaMan I can assure you it's the boiler overflow. I saw the water come out when the water pressure was too high. It's bent back towards the wall so that people who might be standing below don't get hot water dumped on them.
    – Baz
    Apr 9 at 20:04
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I always use diluted muriatic acid to remove rust stains from masonry, but it's a dangerous chemical that can cause burns, blindness and lung injury if misused. It's also known as hydrochloric acid.

There is a product called Bar Keeper's Friend that looks like scouring powder, but it contains oxalic acid, which removes rust stains and is not as dangerous as muriatic acid. You might make a thin paste of it, glop it on and brush it around before rinsing it off. Don't get it in your eyes.

There's also a product called Iron Out that comes in several forms, one of which is a spray-on gel that seems made for this application.

Needless to day, test on an inconspicuous area before attacking the whole stain, and see about re-routing the boiler overflow or you'll have another stain next time the boiler relieves itself.

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  • I've used this stuff amazon.co.uk/dp/B06WP6PY68 Is it similar to iron out?
    – Baz
    Apr 9 at 20:47
  • @Baz No, they are not similar at all. The active ingredients in Iron Out Gel are oxalic acid (removes rust) and fluorosilicic acid (inhibits metal corrosion). The active ingredient in Meguiar's Ultimate All Wheel Cleaner is sodium mercaptoacetate.
    – MTA
    Apr 9 at 21:18
  • Looks like it might be difficult to get iron out in the UK, but I'll keep trying to find it somewhere. Thanks!
    – Baz
    Apr 9 at 21:25
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    @baz Google < oxalic acid products UK > to see a number of products that should work.
    – MTA
    Apr 9 at 22:43

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