Most of my work as a professional decorator is domestic – typically redecorating a room that is already decorated and just needs a bit of a refresh. So, for me, applying wallpaper over bare plaster is something I only do on the odd occasion.
Wallpapering jobs for me usually involve stripping and replacing existing wallpaper, or papering over walls which have already been painted. However, occasionally a householder will have had some work done by builders (e.g. an extension or loft conversion), and I’ll be asked to wallpaper onto walls which have never been decorated before.
Or sometimes I’ll be working on a relatively newly built house, where the walls have had a couple of coats of contract matt only (“builders finish”) and are often dry lined – these share some of the same problems as papering on bare plaster.
The question of how to wallpaper bare plaster came up recently on the Decorators Forum UK Facebook page, and I was staggered by the variety of different replies that the question generated. So, I’m going to try to answer it as fully as I can, including different points of view.
What are the Problems When Wallpapering on Bare Plaster?
Allowing the new plaster to dry thoroughly –
This is really important for the adhesion of the wallpaper – if the plaster isn’t fully dry, the moisture in it will need to go somewhere – it will effectively water down the paste and lead to poor adhesion.
If you think about how to remove wallpaper, we soak it with either a steamer or a mister – the moisture weakens the paste and allows the paper to come away. Wet plaster soaks it too, meaning that the paste may never dry properly (or will dry and then be weakened again), and so the wallpaper will never stick properly.
Additionally, some of the ingredients/chemicals in the wet plaster may soak into the paper, so that even if it does eventually dry out and stick to the wall, it is stained by the plaster. The question is, how long should new plaster be left before papering over it? Opinions varied between one and twelve months!
Personally, I would say “the longer the better”, and certainly leave it for at least a couple of months. It’s better to do a “temporary fix” and paint the walls, then come back and do the papering at a later date, rather than risk papering onto new plaster too soon. There is no way around this issue – none of the “solutions” below will solve this problem, it really is just a matter of waiting.
New plaster (and this also applies to newly built and painted dry-lined walls) is like a sponge, it soaks up moisture like crazy. This makes wallpapering really difficult – there is no “slip”, so as soon as you offer the wallpaper to the wall (“paste the paper” wallpapers) the plaster “grabs” the moisture and the wallpaper sticks wherever it was placed – no chance to reposition it, line up the pattern or whatever. With “paste the wall” papers, the paste just seems to dry into the wall before you can even offer the wallpaper up.
Chemical leaching –
This is not something that I’ve personally ever come across, but it is quite possible that chemicals in the plaster – whether it is fully dry or not – could leach out (triggered by the moisture in the paste) and stain the wallpaper.
Show through –
If using very thin and/or pale coloured wallpapers, there is a good possibility that the colour of the plaster may show through the wallpaper even when it is dry, meaning that white or pale colours in the wallpaper appear to have an unwanted pink or brown tinge to them.
What Steps Can be Taken to Help Wallpapering New Plaster?
Various “solutions” have been suggested; most have their merits (and some their downsides):
Mist coating –
Applying a thinned down coat of paint to new plaster is absolutely essential if you are planning to paint the wall. However, if you are wallpapering, the benefits are less clear cut. Yes, a thinned coat of white paint will help reduce show through, will help slightly with porosity, and may have some benefits in reducing chemical leaching. But it won’t solve any of those issues completely and may create further issues if the paint doesn’t bond properly with the plaster – the paste could then pull the paint away from the wall, causing it to fail.
Applying one or two thinned down coats of paste to new plaster will help massively with porosity and “slip”, but will do nothing to help with “show through” issues if you’re using a very thin and/or pale coloured wallpaper.
Zinsser Gardz –
One coat of Zinsser Gardz (ideally followed by one of size) is a brilliant way to solve porosity issues and chemical leaching issues too.
It effectively seals the wall, making it less “sponge-like” but also stopping anything from leaching out of it. It’s alkali-resisting, so perfect for new plaster (which is alkaline).
But again, it won’t help with show-through (because it dries clear). I’ve also found from personal experience that this works really well on new-build dry-lined walls, which are often “super sponges” and difficult to paper, even if they’ve been painted, but Gardz really helps solve these problems.
Cross-lining the walls (hanging lining paper horizontally) will help with porosity (meaning the paste will dry out evenly and at the correct speed), show through and potentially chemical leaching as well. However, to hang the lining paper in the first place you will probably need to size and/or Gardz the wall first, so it is an additional time-consuming step.
Some people argue that walls only need to be lined before wallpapering if they are uneven to help smooth out the lumps and bumps, and that it is therefore unnecessary on new plaster. There is truth in that argument – newly plastered walls should be smooth enough not to need lining, but lining will always result in a better finish if there is the budget for it, and it will certainly help with show through issues if your finishing paper is particularly thin and/or pale in colour.
Using diluted PVA is a sealer –
No, please don’t. It forms what feels like a thin layer of rubber or latex on the wall surface, which is then really hard work for the next decorator to remove, and it doesn’t actually help solve the above problems.
How To Wallpaper Over Bare Plaster
As stated above, the first and probably most important step is to wait until the plaster is fully dry; if in doubt, paint it for now and come back to it in 6 months. Assuming that it is fully dry, these are the steps I would follow:
Step 1 – lightly sand the plaster to remove any rough or uneven patches, and anything that may have contaminated the surface between the plasterer finishing his work and you starting yours.
Step 2 – remove any dust, either with a vacuum, damp cloth or dusting brush
Step 3 – apply a coat of Zinsser Gardz. This seals the surface, making it much less like a sponge, and much less likely to leach chemicals into your new wallpaper. Allow to dry.
Step 4 – apply a thinned down coat of your wallpaper paste (size). Allow to dry thoroughly. Ready mixed paste is generally better, but follow the instructions that come with your wallpaper.
Step 5 – if the wallpaper is particularly thin and/or pale in colour, I would definitely cross-line to avoid any risk of show-through. Even if it isn’t, I would cross-line of the budget allows for it, to give yourself the best possible chance of achieving a great finish, but given that the walls will be smooth, it’s not essential from a “wall smoothing” perspective.
In summary, that may sound like a lot of work, but we are trying to avoid several different problems when dealing with bare plaster, and while time-consuming, it will make the wallpapering job easier.
On walls that have already been decorated, many of these steps have already been taken by previous decorators; painted walls will effectively have been sealed by multiple coats of paint, and papered walls will have been sealed with previous size and paste. Bare plaster hasn’t benefited from this work, and needs to be prepared properly to avoid the above problems, and to ensure your finished wallpaper job is as good as it can be.
How to Wallpaper over Bare Plaster – for The Decorators Forum UK
By Robin Gofton – Wokingham Decorating Services