Strange effect on a shaving with chalk.

[Mister Zi]

Hello everyone,

I'm renovating a very old room in Southern Italy and have a strange issue with a wall I'm skimming. The house has tuff stone walls, and the existing plaster was in good condition.

I had a small section with peeling paint, minor mold (likely due to a thermal bridge), and some cracks.

My process:

1. Removed all loose, peeling paint.
2. Treated the area with a mold disinfectant.
3. Sanded the surface until the old paint was well-bonded.
4. Applied the first coat of simple powdered filler. It dried perfectly.
5. After 30 hours, I sanded with 120-grit paper.
6. Applied a second, thin coat of filler.

The problem:After 8 hours, the filler felt dry, smooth, and solid to the touch. However, numerous damp-looking patches have appeared. The wall is dry, there are no leaks, and the weather is very hot (30°C/86°F).

I've done other small fills in the same room with the same material and had no issues.

I'm planning a third coat, but I'm worried.

My questions for the experts:

• What is causing these damp-looking patches?
• Is this a sign of an underlying issue, or is it normal?
• Will these patches reappear after a third coat or after painting?

Thanks in advance for any advice
OTHER INFO:

- The shaving was done in the late afternoon.The photos are almost in the evening; I admit the light makes things worse.
Photo A: Wall before
Photo B: First coat, wip
Photo C: Second coat, final
Photo D: Second coat, but with flash and zoom
- The old paint is water-based. The paint on the wall adheres very well. It won't come off with a spatula or paper.
- In the first hand ZERO problems.
- On the second coat, I admit I didn't work the filler enough and didn't let it rest after the hasty preparation. I was late.

Thanks for your time .

 

[Mister Zi]

any ideas?

 

[bobski]

What's the ventilation situation?

 

[Steve Brown]

Second photo, what is this area? Looks like damaged ceiling, possible water ingress from that area?
If that area of wall has had water ingress for a prolonged period of time, and the cause has been fixed + area has dried out. Most often the plaster is compromised and no matter what you do there will always be staining/ moisture re appearing.
The only remedie is knocking it off back the the substrate and starting again.
Make sure that all causes of water ingress are fixed first though.