Stain on Wall

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We have a white, chalky stain on our dining rooms wall, which is traditional lathe and plaster, with painted lining paper (emulsion paint).

Although I have painted over it, this stain has reappeared. Its an external wall, but not in the slightist bit damp.

Any ideas? The ironing board is sometimes set up close to the stains location - could the steam iron cause this?
 
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Hmmm tricky...

Not knowing the history of the wall its difficult to say..I have seen white patches forming on walls beofre and they wernt dsown to steam as far as i know.

I could be a chemical reaction in the wall...caused by alkalis or salts in the plaster/render...it can bleach the surface of the paint...try gving it a coat of undercoat..(oil based) or better still alkali resisting primer...allow to dry for a couple of days and then re coat the wall.

Good luck.
 
To me, what might be happening is efflorescence, only on the interior of the wall instead of the exterior.

Normally efflorescence occurs on the exterior of masonary walls, especially brick walls.

What's efflorescence?

If you have a brick exterior wall, if the back side of that brick gets wet, the moisture will migrate through the brick to it's exterior surface. As the water moves through the brick, it picks up water soluble minerals that were inside the brick. When the water gets to the exterior surface of the brick, then the minerals contained in that water remain behind on the surface of the brick as the water evaporates. The resulting white dusty powder on the exterior brick wall is called "efflorescence".

I'm wondering if what's happening is that steam from the steam iron is condensing on the cold wall nearby, and being absorbed into the latex paint and lime based plaster beneath, where some lime dissolves into the water. That water then evaporates out of the wall, and as the water moves outward, it carries that dissolved lime with it. When it reaches the interior surface of the wall, the water evaporates, leaving the lime behind as a white dust on the surface of the paint.

Steven:

Clean the wall with a damp sponge or rag to remove any of that white powder from the area. Now use some high quality painter's masking tape to stick clear plastic vapour barrier over the wall (or Saran Wrap, maybe).

If the steam iron was the culprit, then the impermeable clear plastic will prevent any condensation from being absorbed into the wall.

However, if condensation starts to form on the back side of the plastic, then that means moisture is coming through the wall from the exterior, perhaps leaking in at a window or due to frost forming inside the wall due to warm air leakage through an opening, that frost then melting and getting the plaster wet. (I'd rule out a pipe leak because it would be foolish for any architect to design a house with water pipes in the EXTERIOR WALLS because if the water inside them froze in cold weather, then the pipes would burst.)

So, if no condensation forms under the plastic, and no powder returns to the painted surface under that plastic, then I'd say the steam iron was the culprit. You can stop that problem by painting that area of the wall with an OIL BASED paint to prevent any condensation from pentrating through the oil based paint into the lime based plaster below.

PS:
Alkyd binder is impermeable to water, or even water vapour. The only way water can get through an alkyd paint, is if the paint is so flat that it actually dries porous. So the higher the gloss of your alkyd paint, the more certain you are that it's not drying porous, but you don't need to go to a high gloss paint to be sure that it's not porous. If there's any sheen you your alkyd paint, you can be confident it's not porous. It's when you get into the flat, eggshell and velvet paints that you can't be sure if the paint is drying porous or not. Paint manufacturers don't all agree on how flat "flat" or "eggshell" or "velvet" are, and I've seen "eggshell" paint that I woulda said was "dead flat", but the manufacturer called "eggshell". As long as an alkyd paint is not so flat that it's drying porous, then it'll be an effective moisture barrier.
 

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