Repair plastered chimney. Which mortar and color?

[Mvdkem]

We have a plastered chimney. The paint on the chimney started peeling, so I took a putty knife to remove the peeling paint to be able to repaint. When I removed the paint on the northern part, some of the plaster underneath began to come off. It crumbled away, it's almost like dust.

The house was built in the 70s. I've read a bit and it seems like I can use C-Bruk to fill in the damaged areas, but I want to double-check here if you have any advice?

After fixing the crumbled parts, I plan to prime it with silicate primer and then paint it with white silicate paint. Do I need to remove all the paint before repainting? Or can I just paint over the existing layer?

 

[Vardagsrum]

It was a nice chimney (or will be again at least!)

C-bruk probably works, but it depends on what type of plaster is currently applied.

One question, however, is why this flaking/erosion occurred?

Is the chimney not used?

I would definitely scrape off the paint, as it cannot be ruled out that it caused the damage. If inappropriate paint (not vapor-permeable or too "dense") is used, it can have similar effects.

Also make sure to scrape off as much loose plaster as possible before replastering.

And for heaven's sake, be very careful when you are up there working! Fall protection is recommended!

 

[Mvdkem]

That was a nice chimney (or will be again!)

C-mortar will probably work, but it depends on what plaster is on it today.

One consideration, though, is why this flaking/erosion has occurred?

Is the chimney not used?

I would definitely scrape off the paint, as it cannot be ruled out that this has caused the damage. If inappropriate paint (not vapor permeable or too "dense") is used, it can have similar effects.

Also, make sure to scrape off as much loose plaster as possible before replastering.

And for heaven's sake, be very careful when you're up there working! Fall protection is recommended!

Thank you for your response! We bought the house a year ago, and the paint was already flaking when we bought it. We have used the fireplace during the winter. The chimney sweep has also been here and confirmed that everything is in order. So my assumption is that the previous owner might have used the wrong paint.

Is there any way I can check what kind of mortar is on it now?

 

[Vardagsrum]

It can be difficult to determine what type of plaster it is.. Unless you're a bricklayer, that is 🙂

Lime plaster behaves a bit like you describe, it can basically be crumbled between your fingers. But weathered cement-lime plaster can also do that..

You can try breaking off a piece and placing it in acid (e.g. vinegar) and see what happens. If it's lime, it should dissolve and bubble.