After I had installed the two plasterboard skins of the outer room ceiling, I next had to seal the gaps between the individual boards, ensuring there was one smooth and airtight surface. To do this, I used scrim tape and plasterboard jointing compound. Essentially, the tape is a self-adhesive porous material that gives strength to the compound when it sets, and the compound is a powder that you mix with water before applying it over the plasterboard joints.
This was a process that I found quite challenging, so I will spend some time describing the tips that I found to make it easier. Here's a picture of my very first attempt at applying the compound - as you can see, not a very neat result at all!
The first thing to understand is the process of how to mix the compound, and what tools are needed to apply it. The key thing here is that the compound starts to dry very quickly (within an hour it will likely be clumpy and unusable), so you need to work out how to both mix it and then apply it quickly and efficiently.
To begin with, after some online research, I found that you can buy a mixing paddle that attaches to your SDS drill. This makes the job infinitely easier as it means you're not mixing the thick and heavy compound by hand! The alternative is a an actual paddle mixer, but these are much more expensive, so I went for the drill attachment.
Here's a picture of the attachment in my SDS drill:
As you can see, I used a scrap piece of plasterboard on which I did all the messy work to avoid bits of compound falling on the floor and then solidifying and becoming hard to remove.
I then used two buckets: one full of water, and another to mix the compound in. First, I poured in the required amount of dry compound into the empty bucket (I would start with half a bucket if you are unsure - this is a good amount to make sure you will use it all within an hour, and you can always mix more). I then added water before holding the mixing bucket between my legs while standing above it so that I could operate the SDS drill with both hands.
After several attempts, I realised that the trick to making the whole job a lot easier is to have exactly the correct consistency of the mix. I found that I often left it too thick, meaning it was lumpy and very hard to work with. The mix should be really smooth when you are applying it - it should feel fairly effortless.
In terms of the actual application, once I had applied the scrim tape to any joints that I planned on filling, I then decanted some of my mix onto a makeshift plasterer's hawk (
this is what a real one looks like), that I made out of a scrap bit of ply and an off-cut of 2by2. Once I was up the ladder and in position, I scraped some of the mix onto my
plasterer's trowel, before applying this to the ceiling and using the back of the trowel to drag it along, scraping of the excess.
There are plenty of YouTube videos that explain the actual "plastering" technique, so I won't try and describe it too much, but I found he key to this part was to understand how the tapered joints of the plasterboard pieces create a natural "valley" in which the compound is meant to sit. This means that you can scrape off level to the plasterboard surface using the long side of the trowel, and you will remove any excess while leaving a neat strip down the seam.
Here's a shot of the ceiling you can see the darker central strip which is where the valley is, and all the surrounding material is excess that would be removed ideally. As this was on the outer room, I wasn't fussed about the finish as it would be hidden by the inner room ceiling, so didn't bother to neaten it up.
One final point to note is that it was significantly more challenging to fill over joins where it wasn't two tapered edges meeting, but two flat edges (e.g. if you have cut a board, or just the short edges of the boards). These are called "butt joints" - and I found it was much harder to achieve a neat finish because there is no valley, so you have to gage the thickness of the compound manually. This is tricky, and it takes some practice to not remove the entire coating while scraping.
On the inner room, I would also have to sand all these joints down after I had applied the compound. For this I used an
orbital sander, with fairly fine sand paper disc. This saved an enormous amount of hand-sanding work, which would have otherwise taken a very long time.
If you are going to be sanding down jointing compound, please make sure to use a suitable mask and goggles as the process will create a lot of dust. I would strongly advise against the cheap paper ones, as they are a nightmare to work with, and will inevitably steam up your goggles. I found a combination of a JSP respirator and a pair of Uvex Safety Goggles worked well to prevent steaming up and were definitely a worthwhile investment.
