Brown marks on new plaster paint

[BrambleyHedge]

Painting new plaster in our extension and did a mist coat as plaster looked dry enough. In places (where old house exterior was) we have a few brown stains coming through. Not sure if to treat with stain block or to remove the mist coat with sugar soap (if that works) and wait for it to dry out...which could take a while if it is the old exterior wall drying out. Any thoughts?

show us some photos please

is it near a chimneybreast

or an old water leak?

or an arris?

I wasn't sure what an arris was but having googled it, that is probably it. Two pictures. One is old external back corner wall and one is old external back wall next to party wall (semi). Wall was single skin and now has plasterboard on it.

Pics

The one with the ladder was a bit more than 'mist' as husband did different mix...

Should also add we have inadequate heating currently

that's a damp mark.

when was it plastered, and when did the roof go on to keep the rain out?

is there a pipe in the wall, or a shower or sink on the other side?

if you are painting damp plaster, use Dulux Trade Supermatt or similar, it is a porous, non-vinyl paint allowing the wall to dry out through it. It is not very durable but you can paint over it when full dry.

Other brands will be marked "paint for new plaster"

you can speed up drying by blowing a room or desk fan (not a fan heater) on it

open the windows during the day

all the time the windows mist up, it is still drying.

"you can paint over it when fully dry."

I mean, when the wall is fully dry

not just the paint.

Thanks. It was a damp wall beforehand but not due to any roof issues as we looked at that. The new side roof went on a few months ago and was quick with no rain. Our bedroom and ds room are above and where the radiators are there is no damp coming through so I think it is probably residual as the wall has had condensation issues. We have Dulux matt but try supermatt like you suggest. Thanks!

I've lost track of which bits of plaster were done when but probably 3-4 weeks. It looked pale enough but we were probably too keen. The rest is all ok.

Actually it is not likely to dry out as the extension is ground floor pitched and the wall above is still single skin rendered so will need to have a think about stabilising that.

That was supposed to be a sad face

Thanks for the advice @PigletJohn