One of the biggest problems with plastering is making sure the preparation is complete. This article will give you the best advice in preparing walls for plastering.
There’s a very common saying that I’m sure you have heard: Failing to prepare is preparing to fail! Its the same case when skimming. You always need to make sure everything is prepped correctly!
We’ll show you exactly what to do and show you the materials you need to buy to get it done. There are a few subjects we have to cover before you start. Lets do it:
High Suction/ Low Suction
When you plaster onto a wall you need to check the walls for suction. Your probably thinking what does that mean and why does it mater? It is a really important subject to get right.
All walls, whether its bricks or plasterboard, have a degree of suction. For example bricks have a really high level of suction where plaster board is really low. Learning to control this suction is what you need to learn.
When you use finish plaster the water in the mix gets drawn from the walls as it dries. Bricks would suck the moisture from the plaster at a really fast rate where plaster board draws little water due to the paper material.
If you plaster on a high suction background it gives you little time to skim and get a finish. This is why we need to control the suction. It gives you more time to get the plaster perfect. There are some methods you can follow to get it just right.
Preparing walls for plastering
Lets say you have a wall in your house you want to plaster. You need to strip all the wallpaper and make sure the wall is clear. If you plaster on paper its falls from the original backing and peels the plaster off with it.
So the walls clear but now what do we do? Follow this routine and you should be clear from any problems. Here’s a quick summery:
- Strip Wallpaper
- Wash the walls down
- Check your walls
- Skrim tape any cracks
- PVA/ Blue Grit
1) Strip Wallpaper
You need to strip all the wallpaper and make sure the wall is clear. If you plaster on paper its falls from the original backing and peels the plaster off with it. Make sure you get all off. You dont want any hidden problems because of a bit of wallpaper.
2) Wash The Walls Down
You might not need to do this every time but I’d recommend doing it. This process will remove any unwanted grease or grime on your walls and if we don’t do this the grease effects the plaster. I’ve got a story where it went horribly wrong for me.
We re-plastered an old ceiling once which looked fine. I didn’t wash the surface (again, it looked fine). I noticed that a few areas were sagging when I was applying the finish but I thought it would be fine when completed.
We had to go back to the job the day after and realised there was a bubble in the finished plaster. When I looked closer I saw lots of micro cracks. After 5 minutes the whole lot was on the floor!
There was grease hidden and the plaster didn’t adhere to the previous surface. I learned my lesson the hard way! You can’t always see problems on the walls so its always good to give it a wash with sugar soap. It cleans the walls down ready for the next step in preparing walls for plastering.
3) Check Your Walls
The first thing you need to do it check for any loose plaster. If there are any areas that are cracking or peeling make sure you scrape it off. If you plaster onto a unstable walls you could risk it all falling off. I’d also Remove any peeling paint.
You also need to check to see if the previous coat of plaster is solid. We can’t plaster onto a wall that is drummy or unstable. If you tap the walls you can hear when plaster is solid to the brick. It doesn’t move, doesn’t sound hollow and definitely doesn’t fall away with pressure.
Cracks are signs of weakness in the walls so tap around any areas that seem unstable. if any sections sound hollow or move you need to hack the old plaster off – you cant leave it. It will cause way to many problems further down the line.
Hopefully your wall is nice and solid. If some areas need to be hacked back then you will have to fill that with Bonding. I use British Gypsums Bonding thistle undercoat bonding. Its a brilliant product which can be plastered onto brick and many high suction backgrounds.
If you have smaller holes id just use some filler before you start skimming. Its a lot easier!
You will also want to unscrew your light switches and plug sockets. Its better because it saves getting plaster all over the fittings. I like to place them safely in plastic bags so no falling plaster hits the electrics.
This just makes your life a lot easier when plastering
Once you have everything cleared and you have a solid wall with no loose materials your good for the next part.
4) Skrim Tape
Once you have a your walls scraped and clear you need to prevent any problems returning. You might see a few cracks on the walls. If thats the case you need to apply scrim tape where the crcaks are showing.
Its basically a mesh that sticks directly onto your walls. We do this because if there is any movement it keeps the plaster in tact and prevents any cracks appearing onto your finished walls. Its a really good product that allows for movement.
When your plastering on an existing wall you need to make sure you control the rate of suction. The best way of doing this is to paint or roll your walls with PVA glue. But why do we do this?
The PVA acts as a primer. It’s only really used to stop the plaster drying too quickly. This prevents the walls from drawing all the moisture from your plaster making it stronger. Its also gives you more time to get that perfect finish.
You should always mix the PVA with water. The instructions are found on the tub but I like to use 3 parts water wit 1 part PVA. To be honest though I find some PVA really weak so I have used 2 parts water to 1 part PVA previously.
You simply paint or roll the PVA straight to your walls but you might sometimes need 2 coats. If your plastering over Artex for example I PVA the ceilings with 2 coats. This is because Artex has a high rate of suction.
The double coat increases the efficiency of the PVA providing a better rate of control over the suction. You simply apply the PVA, wait for it to dry and apply the second coat. If your starting out I’d always use 2 coats – its makes skimming so much easier with a decent application.