Walls painted, unheated for years - now there is staining.

Comprehensive renovation of an old farmhouse. Gas, water, electricity – everything has been redone. For 3 days now, we finally have heating as well!! Our walls, painted just over a month ago, are now becoming heavily spotted (as can be seen in the photo). We ventilate every day and have the radiators set to level 2-3. Since today, we have also set up dehumidifiers. We are aware that it takes some time for moisture to escape from such an old house.

We are aware that it takes some time for the moisture to escape from such an old house.

You are not asking anything now. … so it should be clear to you that you first turn on the construction dryer, then check the humidity as well as the wall moisture, and only then paint.

You are not really asking anything now. … so you should be aware that you first turn on the construction dryer, then check the humidity as well as the wall moisture, and only then paint.

That was not clear to us - no. The stains already go away well after only 24 hours with the construction dryer. Thank you!

Heat the place up finally and dry it. As ypg wrote, you cannot paint on a damp wall (see the instructions of the paint). You will have to wait and see if the coating will need to be renewed.