Stains on new plaster

[erikwoolf]

Lime plastered around the dining room this week. In a couple of places big yellow stains have now appeared after 2 day. Any idea what could it be? They were not visible before plastering. Will washing the wall with sugarsoap help prevent them from appearing at the rest of the walls?

 

[Toby Newell]

I defer to people with more experience of lime plaster but as a basic scientific material observation...

A. Sugar soap = an alkaline detergent that breaks down and removes oils, fats, waxes and grease.

B. Your discolouration is almost certainly not caused by oils but rather by water soluble salts in the substrate behind the lime plaster which have dissolved into the plaster, wicked through to the surface and come out of solution ( precipitated) on the surface, in a pattern like on blotting paper.

C. Thusly sugar soap is likely to have limited effect, you may see a slight improvement even witha clean water but the stain is most likely fixed into the plaster.

D. It is possible there is nicotine staining but that usually breaks through paint and not plaster.

E. Having said all that there is no harm in first trying sugar soap! It is alkaline and thus will not damage your plaster.

F. The stain may break through your paint or it may not. Stain blocking paints tend to be less breathable than normal paints. Lime wash is especially prone to stain bleed through. This is unfortunate! You will have to experiment. Zinsser used to make a shellac based stain blocker, this is somewhat breathable but check the product information I believe they do a more modern version now which may be less breathable.

G. FYI you can make your own traditional stain block by mixing max 10% white mixol universal tinter (exact same pigment in commercial paints) with Mylands Special Pale Transparent Polish (shellac)

H. I did this myself in the bedroom of a rockstar (Radio head XD) and it worked very well. You may need to apply more than one coat.

I. Always try the easiest solution first, ie clean water, then sugar soap, then coat with your preferred paint. It may be OK. Only escalate to trying stain block if all else fails.

Best of luck!

 

[a twig]

I’d agree that it looks like something being pulled from the wall behind, quite possibly salts.

What are you planning on finishing the walls with: line wash, clay paint, something else?

 

[erikwoolf]

We were planning to paint over with clay paint. These walls were covered with 2 different layers of wall paper before.

 

[Lacan07]

Its an internal wall so unlikely to be due to damp?

that colour looks like the distemper some of my walls were originally coated with

 

[erikwoolf]

Its an internal wall so unlikely to be due to damp?

that colour looks like the distemper some of my walls were originally coated with

This is an internal wall, it is weird as it has appeared in some places and not in others.

 

[stuart45]

What's the wall built with?

 

[erikwoolf]

What's the wall built with?

Solid brick and lime, originally.

 

[Lacan07]

Solid brick and lime, originally.

I assume you lime skimmed these walls over the original lime? I'm wondering if the acidity of the lime has raised a previous finish that wasn't visible. Like I said, that yellow looks very familiar.

Did you float the skim flat at all before polishing, sometimes that can have the effect of reviving whats underneath.

 

[Toby Newell]

I assume you lime skimmed these walls over the original lime? I'm wondering if the acidity of the lime has raised a previous finish that wasn't visible. Like I said, that yellow looks very familiar.

Did you float the skim flat at all before polishing, sometimes that can have the effect of reviving whats underneath.

Pedant patrol.

It is perfectly probable that the stains are precipitated salt solutes that were leached from the underlying substrate.

Lime is indeed caustic, thus reactive.

It is also famously very un acidic.

As in very heavily alkaline. Around pH12 when wet.

 

[erikwoolf]

I assume you lime skimmed these walls over the original lime? I'm wondering if the acidity of the lime has raised a previous finish that wasn't visible. Like I said, that yellow looks very familiar.

Did you float the skim flat at all before polishing, sometimes that can have the effect of reviving whats underneath.

Yes it is a very thin lime skim that we did, apart from the areas that the old lime was removed and patched which coincidentally or not don't seem to feature any stains. What ever this is it definitely came back from the dead because of the new skim coat.

 

[Lacan07]

Pedant patrol.

It is perfectly probable that the stains are precipitated salt solutes that were leached from the underlying substrate.

Lime is indeed caustic, thus reactive.

It is also famously very un acidic.

As in very heavily alkaline. Around pH12 when wet.

Yes, you're right, salts is perhaps the most likely explanation.

I think clay paint may be you're best bet for a finish Erik

 

[a twig]

I had some staining around an old chimney breast, two coats of thicker than normal lime wash (about the consistency of single cream) and then two coats of clay paint did the job of covering everything