Best DIY way to tackle this...

digannio

I have had an issue on an area of  plaster on an external facing wall. Originally there was a tide mark on the internal plaster just below the window sill with some blistering of the plaster which I believe to be hygroscopic salts. The outside wall is some kind of cladding stone and always gets the prevailing wind and rain.

I applied a waterproofing cream to these cladding stones which seems to have stopped water from being transferred through into the internal plater. The water/tide mark on the inside plaster has now gone despite some intense rain hitting that wall recently so the waterproofing cream seems to have worked.

My question now is what to do about two small areas of salt blistering that remain on the plaster. I'm looking for a way to treat/remove any residual salts that may be left in the plaster. I gather these salts can be stubborn and not sure that simply sanding down and repainting will stop it for good. Doesn't feel or look like any damp in plaster now, just these two small areas of salt deposits. Pics show the external wall that has been treated with the breathable cream and the internal blistering. There are no problems above with any gutters to cause problems in future and sealant around window is good.

Mr.Generous

I'd go for sanding, if you use liquid the salts can go into sollution and re-crystalise again bubbling more paint. After sanding I'd roller the area with some Zinsser stain block - a couple of coats to be sure - then overpaint as normal.

Don't bother with any water based stain block, they won't work.

digannio

Thanks, I'll have a look at this zinsser stuff. I wonder if it would be of benefit to use a salt neutraliser as well after sanding the area down?

Eldi_Dos

I would be inclined to take out the first row of paviours,put in a thin edging strip dig down a depth then a bed of gravel between wall and paviours.
Check where your DPC is in relation to the paviours.

digannio

What I was concerned about with that approach is ending up with a gravel trench next to the wall that just fills with water in heavy rain (the soil underneath is largely clay). At least now a lot of the water runs over the blocks and away from the house.  Also the waterproofing cream I have used on that wall really does seem to have stopped moisture getting through the wall as the tide mark on the inside plaster has completely disappeared since I used it. All I'm left with are two small areas (each about 3ins long) of residual salts on the plaster surface just below the window sill which are not growing in size. The DPC is about 13cm above the blocks.

FreeBear

digannio said: The DPC is about 13cm above the blocks.

130mm is plenty to stop most of the splash back when it rains. As the damp patches are just below the window, it is highly unlikely that it is a result of the paving being too high in relation to the DPC. Instead, I'd be looking at the condition of the mastic either side of the exterior sill.

digannio

I replaced the exterior sealant below the sill and the old stuff was bone dry behind when I took it off. The side sealant looks good. 

stuart45

If that's a cavity wall there must be a fault in the construction to allow moisture through to the inner skin.

digannio

It is a cavity wall and I was thinking maybe the retro fitted insulation beads or some debris had allowed some moisture across to the inner wall, albeit in just a small area. But it does appear to have dried right out now since I used the cream on the external wall. I guess I am trying to sort it without taking the whole wall apart to solve what appears to be a small problem area on the plaster that has never got any bigger in the couple of years I've been here.

stuart45

Bricklayers sometimes used to close the cavity at the cill by cutting 4 inch blocks to lay flat across the cavity. Without a DPC, this occasionally allowed moisture to track across to the inside.

FreeBear

digannio said: I guess I am trying to sort it without taking the whole wall apart to solve what appears to be a small problem area on the plaster that has never got any bigger in the couple of years I've been here.

You could try lifting the window board on the inside - Done that here with a couple of windows to find that the cavity is open underneath. One had a bit of roofing felt laid in with great globs of mortar chucked in on top (builders that did the extension were cowboys). The rest have a row of bricks closing the cavity off - All easy enough to drill a small hole through so that a camera can be inserted. Something like this would do the job - https://www.amazon.co.uk/Endoscope-Flexible-Inspection-Camera-Pipeline/dp/B0CJT2RNKC/ref=sr_1_4?th=1 (also available from other sources).