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I moved to an old house south of Germany. For various reasons (mold, ugly looking, etc.) want to paint the walls from scratch by scrapping all the wallpaper and paint. I started from one room, I scraped all the wallpaper and old paint by using a spatula. I will sand all the wall surfaces to remove all the existing residue of wallpaper and paint. (glue etc.)

In the end, I will have the following surface as a starting point:

sanded concrete plaster wall

It is a thick plaster layer (like concrete) over bricks.

I want a nice finish and a solid & durable coating against the mold forming so I don't have to build my walls every year. (humidity is relatively high)

I read a lot on the internet about how to build a good wall finish. But still, I don't have a concrete idea of how should i prime, plaster, and paint the walls in the right way.

There are a couple of questions in my mind:

  • Do I need to prime the raw surface before plastering? If so, which material should I use and how to apply it? There are articles suggesting applying PVA before the plaster to control the moisture from moving from inside of the wall to the plaster/paint surface.

  • Is sanding over the plaster a bad idea? I read some comments about the plaster should not be sanded at all. I was thinking to sand the plaster after it dries to make it perfectly flat because I am not very experienced in plastering and most likely will end up with an uneven wall surface.

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    I'm no expert on this, but be careful, you don't know what's in the sanding dust, use good dust control, wear a respirator. Next: I had a friend who needed to remove a lot of wall paper and he rented a steamer for that purpose. It softens and loosens the wall paper making it much easier to remove. The added benefit is it moistens it, which keeps the dust to a minimum.
    – George Anderson
    Jul 15, 2022 at 13:15
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    @GeorgeAnderson thanks for the suggestions! I bought a good respirator once I realized basic dust masks are not sufficient. I wanted to give the streamer a try but in my case wall paper was painted a couple of times and even warm water spray was unsuccessful in penetrating the thick paint layer. So I had to scrap all the wallpaper and paint with a spatula.
    – Yarimadam
    Jul 15, 2022 at 18:02
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    Why are you concerned about mold? If there is mold now, do you understand how and why it came to be there? That's important information. Also, the sanded area seems rough which leads me to believe the existing surface is very hard. So if you want a smooth surface you should coat it with something. And if you coat it with the right product, it won't need sanding. Which products to use is something you should determine from local experts, including a local store.
    – jay613
    Jul 18, 2022 at 11:05

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+100

Old houses in the UK (where I live, but I would imagine the same it true elsewhere) often have sand & lime plaster, rather then the more modern gypsum-based plaster.

This stuff has a nice shiny polished surface, but once you've sanded that very thin top layer away, you're left with a soft gritty surface that no amount of sanding will smooth.

Rather than sanding, I would wash the walls, and then use a scraper to remove the softened paste. You may need to do this a couple of times to get rid of all the residue. I have successfully done exactly this in the room that I am currently sitting in, in our 1950s house.

Once you've done this, no priming should be necessary, and you can paint with normal paint for internal walls.

Obviously, if the surface, after removing the paste, is not smooth, you will need to coat it with something. The best finish would be obtained by getting a local plasterer to apply a thin 'skim' coat of new plaster.

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