Mystery stains on external wall under concrete sill

[trilby]

A year or so ago I stripped the blown Sandtex masonry paint under our downstairs bay window, revealing some staining that looks like it had come from the concrete window sill. I painted over it with a different masonry paint and within a few weeks the staining had returned.

Whether it was new staining or the old staining somehow showing through, I couldn't tell. However it's only on this one side of the bay.

Attached are some pics. The white gap between the stains is the non-blown Sandtex which I left. It doesn't stain, however I don't just want to repaint with Sandtex only to have it blow again.

If anyone can help identify what's causing the staining and how to stop it, I'd be very grateful.

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(Originally posted in the 'Building' forum, I think this is the more appropriate forum for this post but don't know how to delete the last one.)

 

[Chri5]

Christ that's an absolute pigs ear.

Staining likely due to sills being very poor, water sitting on them and creeping behind sills at the (obvious) join points.

In all honesty there is a few weeks work to straighten the mess up, and I suspect most of the lower wood verticals and the horizontal wood that butts to the sills is going to have rott and damp.

You could start by cutting the joints on the sills and remaking the joins between each one with Toupret. You then need to layer Torpret from the back of the stone sill to allow all water to run off the sill, rather than sit on sill or run back in and under window wood.

Then seal all back edges with sikka flex and re paint.

 

[misterhelpful]

@chris5

I suspect most of the lower wood verticals and the horizontal wood that butts to the sills is going to have rott and damp.

Are we not looking at UPVC windows and a stone sill here??


@trilby
Looking at the whole area under the bay, and even the bottom section of the whole house, I can see that there is some discolouration compared to the paint above sill level. I wouldn't be surprised if the whole section has had some kind of bitumen type paint/damproofing applied at some point in the past and that could be what is bleeding through. Do you know if this is the case?

Has the very white section below the sill had the render knocked off and been patched up?
If it has, it would explain the difference in colour as there would be nothing there to bleed through that section. This could also help to explain the darker spots in the corners where moisture from the new render has caused stronger bleeding.

If none of this is the cause, it could just be water penetration from damaged the area around the sill, which really does need to be repaired before you do anything else.

If the render is solid and in tact, you can scrape off any loose paint and then apply a good quality stain blocking primer like Bullseye 1-2-3 Plus all over the wall, or possibly B-I-N if you just want to spot-prime the worst areas, although the latter is harder to apply, is more likely to 'flash' through the topcoats, and isn't really suitable for the whole area. (Some would use thinned oil based undercoat but I personally preferred to avoid that on external render). Otherwise, you will need to remove all loose paint and render before patching up and re-painting.