Stain on outside render

[george121212]

I have removed some sleepers that were up against the rendered outside wall. Previous owner put them there but removed as I am concerned about damp. Going to get exposed areas blocked.

A few marks have been left where the sleepers rested against the wall. I have tried 50/50 bleach water mix, scrubbing with a bristled brush but cannot seem to remove it.

Would sodium hypochlorite work? (Have already tried bleach) or is it a touch up paint job?

Pictures show before and after sleeper removal.
Thanks

 

[tell80]

You have damp thats perhaps penetrated into the render.
Do nothing for a couple of months but observe - if it gets worse then Hack off the patch, and re-render with a plastic Bellcast. Forget about the bleach etc.
I dont think the render has been finished with the correct Bellcast.
No air bricks shown, are the floors solid?

 

[george121212]

Hi house is 1930s/40s so solid floor.
Got a feeling it could be rust/tannin so will try those removers if not then like your plan.

 

[DiyNutJob]

Don't be pussy footing with the bleach. Go full strength with stiff brush. After dry, stiff brush with water. After, repaint to cover all stains. Now, you will be ready to observe if the problem is still alive. Save the before photos for later comparison.