Anyone seen anything like this before?

[floorboard10]

Had my walls recently skimmed, put a mist coat on then 2 coats of dulux vinyl matt emulsion on, about a month later, damp marks started coming through. the picture with the hole is where the waste pipe is boxed in, i cut a hole to check for leaks and everything is dry?? so a bit confused. the other picture is of a internal wall, There must be moisture present on both sections. as when i put an electric heater on the marks it disappears but reappears within a few hours. im not exactly sure if it is damp as there are dry spots in the middle of the patches. the skim was left to dry for about a month so dont think its that, anyone have any ideas????

The skim wasnt sanded not sure if this has had any affect

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[NickB_99]

im not exactly sure if it is damp as there are dry spots in the middle of the patches. the skim was left to dry for about a month so dont think its that, anyone have any ideas????

The skim wasnt sanded not sure if this has had any affect

You shouldn't sand a skim, so that sounds fine.

I'm not exactly sure, given you suspect waste pipe is not leaking.
A possible suggestion - condensation from the moisture in the house?
Looks a bit uniform for a leaking roof etc.
Given it's a waste pipe inside, there will be an external vent at the top to the outside air, which may lead to air a bit cooler around the pipe.

If you search the forum, there is a test you can do sticking some polythene on the surface to where condensation is coming from - do a search. Hope it helps.

 

[stevethespreader]

your saying that damp marks are coming through the wall and that your not sure if it is damp do the walls feel damp to the touch? also we know that one wall is boxed in, what is the other wall is it solid dot and dabbed? or what?
"EDIT" is the room next to a kitchen ? and is there any ventilation?

 

[floorboard10]

Hi the wall is cold but doesnt feel damp at all. the wall picture is solid brick (internal) no bathroom or anywhere condensation could be made, cooker shower etc. the bit that is boxed in is the waste pipe but is boxed in with wood then plaster board and plaster on top of that. i cut the hole in to see if it was condensation but mark still appeared. Loads of ventilation. On hotter days the marks do look slightly better if that helps. Just find it strange where there is no marks within the "damp" patches so im thinking i wonder if the plaster wasnt fully dry and certain bits did dry. No heating was on during the drying process

 

[stevethespreader]

if the walls are dry you have nothing to lose by giving them another coat of paint use a different paint and see if the dark patches reappear through the new paint, thats what i would do if the walls are dry