ᐅ Chimney/Flue: Repair the plaster and smooth it out with a skim coat.

Hello,

In the attic storage room, there is a section of chimney flue where the plaster and paint are peeling off. The chimney (the house was built in 1972) is constructed with Plewa elements, and at some point, a stainless steel liner was installed inside.

The flue is currently out of use (since the heating system was changed from oil to a heat pump). Previously, the attic was an unheated roof space, but now it is part of the thermal envelope. The stainless steel liner is still open at the top, but I plan to seal it soon. I would like to repair the plaster, apply a smooth filler, and repaint it so that the flue could be put back into use if needed in the future.

Which type of plaster or filler would be best suited for this (loose plaster will, of course, be removed beforehand)?

Hi,

Peeling plaster on the old Plewa stove is almost always a moisture and temperature issue, especially now that the attic is heated. It is important not to use gypsum plaster here, as it is unsuitable and will cause problems again as soon as you start using the stove. Remove any loose plaster and stick to mineral-based products—a lime-cement fine plaster or a special chimney filler works well. These are vapor-permeable and temperature resistant, which is crucial in this case. Apply the plaster thinly and avoid over-smoothing, otherwise you trap moisture inside. Afterward, paint with silicate paint, not standard emulsion paint. While emulsion paint may look nice, it usually only holds up cosmetically. Also, I recommend closing the stainless steel pipe at the top soon, otherwise moist air will be drawn in and the problem will start again.

Good luck!

Nauer schrieb:

Hi,

Peeling plaster on the old Plewa chimney is almost always a moisture and temperature issue, especially now that the attic is heated. It’s important not to use gypsum here, as it’s unsuitable and will cause problems when the chimney is used again. Remove loose plaster, stick to mineral-based materials—a lime-cement fine plaster or a specialized chimney filler works well. It should be vapor-permeable and temperature-resistant; this is crucial. Apply it thinly without smoothing it perfectly flat, otherwise you trap moisture. Finish by painting with silicate paint, not a standard emulsion paint, which might look good but usually only holds up cosmetically. Since the stainless steel flue pipe at the top is still open, I’d recommend closing it soon to prevent moist air entering and causing issues again.

Good luck!

I have to disagree. The chimney is out of use, so it is basically just a wall like any other. (And if reused, a stainless steel liner has been installed, which eliminates the old chimney’s temperature and stress issues.)

That aside, I would also use a plaster similar to what was originally there (cement or lime-cement). If you want to stay mineral-based, lime putty finish is an option (you can apply and sand it like gypsum fine filler).

@nordanney @Nauer
Thank you very much for the feedback. For large chipped areas, I would repair them with a lime-cement plaster. How should I pretreat the existing substrate afterward before applying a smooth filler (I still have Pufas LF and Pufas silicate fixative available)?

The plan would be as follows:
- remove old loose plaster
- repair damaged areas with lime-cement plaster
- pretreat the substrate (Pufas LF or Pufas fixative)
- apply a thin, smooth layer of filler with Pufas silicate lightweight filler (would that be suitable? – I also still have some)
- paint with silicate paint

Fits well. At most, apply a primer as you suggested – it certainly won’t hurt.