Skimming over dusty lime

[my_name_is_slim_daisy]

Hello all and happy new year!

I’m onto my next wallpaper project and unsure if the approach I’m taking will work. I’d be grateful for any advice.

I’m lining over lime walls again and just want to be sure the wallpaper won’t lift at the seams because of poor adhesion and poor skim/weak plaster.

I have a 2m x 3.5m wall. After doing a tap/scratch test I ended up scraping off around 50% of the skim - so the wall is very patchy right now! In some areas it needs a 1mm skim, and 2mm skim needed in others. What I see now is a lot of dusty lime (see picture).

My plan is as follows:
- use Zinsser Gardz to stabilise the lime and remove dust. I would use Earthborn Wall Glaze but it is very pricey in comparison.
- use Toupret fine filler for the skim and embed scrim tape.
- wallpaper over this with grade 1000 lining paper.
- paint with F&B or similar. I had a bad experience with the cheap Deluxe, and their Contract Matt isn’t great either.

My questions are:
- should I use a setting compound in the first skim?
- is there a good rule of thumb for applying scrim? The wall is just very patchy and it is a bit confusing figuring out what I do and don’t need to scrim.

 

[montyjohn]

I wouldn't want to use Zinsser Gardz. It's not breathable. Given the condition of the plaster it likely does get exposed to water.
Toupret fine filler isn't very breathable either.

I would be brushing off the loose and applying a lime putty fine plaster mix as a skim coat.
If you can't brush back to a stable layer then it likely need replastering.

Also make sure you are using breathable wallpaper. Or even better skip this step.
I assume the Farrow and Ball paint will be, but check.

Do you know what the old filler stuff is?

 

[my_name_is_slim_daisy]

Is there a lime putty fine plaster mix that you recommend? The older filler is polyfilla I think.

As the surface is dusty lime is there anything else I could use to prepare the surface for skimming?

Because the wall is so patchy and there’s wallpaper on either side - I most likely will use lining paper. At some point in the future I expect I might go back to brick to add insulation, and at that point I would like remove wallpaper altogether

 

[a twig]

Limewash is a great and cheap way to stabilise dusty lime plaster.

Beeck Fixative Silicate could be another option

 

[Cubist]

When applying wall paper, of any kind, it's often recommended that you 'size' the walls immediately or very shortly before you offer up the wall paper. Typically the sizing substance used is the same wallpaper paste you intend to apply to the back of the paper when you lay it up. In some cases when papering dry and dusty plaster walls it is strongly recommended that the 'size' is applied in two coats.

The first coat is allowed to dry almost completely and this operates to aid consolidation of the plasters structure and also to reduce its ability to absorb further moisture. During this stage there is a possibility/risk that already loosened surface skim coat plaster may separate and fall away but this such damage can be easily repaired ahead of the next real stage. This first coat also serves to prevent dust which may reduce the adhesion capabilities of the pasted paper when it's applied.

The second coat is applied immediately before laying up the paper, to limit the 'grab' - induced by the rapid absorption of water - from the paste on the papers back as it is first applied to the wall and to then aid the wholesale movement of the paper to allow close alignment with neighbouring paper strips and to allow you to easily lift and move large sections to eliminate say a wrinkle or crease.

In terms of your plan;

• if the plaster is sound and the damage you have found/created is superficial to the skim coat then fine fill (the Tourpret product is suitable) and smooth the plaster repairs - I'm not sure why you would want a 'skim tape' unless you have to negotiate a corner somewhere -,
• apply a first coat of the paste to the plaster and brush out any lumps and allow to dry,
• cut and paste the lining paper and set aside while you apply the second coat of size to the wall,
• then layup the paper and manoeuvre into the required position and trim to size where needed,
• leave to dry for a few days and then paint.

If, after you've laid up the paper, you find that an edge has failed to grab the wall you can slip the end of the pasting brushes bristles under the edge and use some side to side movement to apply some more paste to the wall and the back of the paper.

 

[montyjohn]

Is there a lime putty fine plaster mix that you recommend?

I would just use lime putty mixed with fine sand or marble dust. Don't go mixing it with sand from B&Q, you'll need to get washed, clean, sharp, fine (as in less than 0.5mm) stuff.
I just don't know how well it will stabilise your crumbling plaster. I would want to remove all the loose before doing this.