Black spots, need advice

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I applied Ronseal's exterior wood protection on an Oak wood surface because the builders left a small piece of the staircase untreated (and stupidly, I decided to fix it) and after a few days, some black spots formed (see pictures: first picture is after the application and second picture is what it should look like...). I gather I used the wrong treatment.
Is there any way I can get rid of the spots and what kind of oil would be best to apply? (Danish Oil?) I sanded down the surface a little but the spots seem to go deeper.
Any ideas?



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did you use wire wool/brush ??
tannin in oak causes black when reacting with steel
 
I think I used a regular brush (pictured). Any way I can rectify this? Or do I need to get the builders to replace the whole part??
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be aware of the fact that bleaching may cause the surrounding unstained wood to lighten up as well, creating a donut halo effect. To prevent the halo, you might mask off the first 6 inches around the stained area or apply a bit of new finish there (that may be sanded off). Although you might try bleaching with peroxide, I would advise against using Clorox, because it damages the wood.
 
Thanks. I am trying toothpaste first (there's a youtube video that claims that it gets stains out of oak but my stains seem to be deeper).
Any advice on what would be a good oil for oak tops to protect without staining/damaging it for future? I ordered this: https://www.amazon.co.uk/Top-Oil-30...=UTF8&qid=1474037163&sr=1-2&keywords=osmo+oil And also have Danish oil
The builders used Bona products I think. We have lots of oak (including engineered hard wooden floors are also from oak) and it seems to be difficult to find the right products to treat/protect it without staining it. There are a few bits which show slight discolouration (see middle of photo, it's grey and looks as though the treatment has worn off or never been applied properly). It's been about 3 years since the floor has been installed/treated.

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You could try oxalic acid which is specific for the black marks caused by iron stains in oak. It's also good for removing rust stains from most surfaces.
 
Those black spots are an organic growth & you need to kill it.

The only thing for sure, in the first instance, is you need to purge your mind of 'Ronseal is good' & gather any remaining product that bears its mark & chuck it.

Once you have done this, have a chat with an oak specialist, someone who REALLY knows their oak & beg them to extend their knowledge by tackling your problem.

Me, I'd start by wiping down with neat Jeyes fluid to see if that would kill it.
 
Jeyes fluid on an internal bannister? Are you sure or missing a smiley?
 
I see no reason why that would be organic growth appearing after a couple of days just because it has been varnished with ronseal.
As Big Al says that looks like iron reacting with tannins, are you sure that there hasn't been any iron involved, grinding nearby or anything like that or a rusty ferrule on the brush.
oxalic acid should remove it if that's what it is
 

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