Afternoon everybody, just after a bit of insight as to why I’m having this problem.
So, I recently re-skimmed my spare bedroom, it’s almost all dry now, just a few damp patches here and there.
I wanted to get the new electric radiator all fitted so that after painting I can just re-fix the brackets and connect everything back up and not potentially scratch the paintwork or leave pencil marks.
I drilled my first hole for the brackets and the skim coat around it blew and cracked apart. The immediate area around it can also be hacked off fairly easily with just a filling knife and a bit of elbow grease.
This wall was the last surface in the room to be skimmed and although as I said there were some damp patches, the spots where I had to drill for the brackets were all dry.
I decided to drill another hole in the first wall I skimmed, which had no damp patches at all. The wall with the brackets is also an external wall and I thought that maybe it was still a bit damp below the surface and with the cold and wet weather maybe that just needed to dry out a bit still.
But no, same problem on the other wall which is only a partition! Please see the attached pictures.
A bit of background info:
the walls are 3” block internally, 2” cavity filled with some white insulation foam and then a single skin brick wall externally. I believe the house was built in the early 70s and the blocks almost look as though they have small pieces of wood fiber in them. Not as soft as a modern thermalite but easy enough to drill through without a SDS on hammer drill.
The walls were rendered with what looks like a somewhat coarse mix, quite a few big chips of stone and chalk in there.
They were them skimmed with a gypsum finishing plaster. When I first moved in it became clear early on that this would be a problem for me, as again it was very poorly adhered in places and could be scraped off without much effort at all. It wasn’t that bad everywhere, but as I had no way of telling where it would be like this I decided to hack it all off and re-skim everything.
So I initially coated the walls with a mix of 1/3 watered down PVA. This was really just to keep the dust at bay while I got on with other things like door linings and window boards.
When it came time to start skimming, I took the advice of a Youtube plasterer and coated them in neat PVA. I let it all dry completely and I figured this would be okay as the PVA would re-emulsify when wet and stick quite well to the old render that was still fairly porous after the initial 1/3 coat of PVA. Some areas I had also blue gritted, as I had parts of the render to patch up with bonding and after letting them dry. I gave them another coat of blue grit and then a coat of neat PVA, as I felt this would rough them up a bit and be a similar match in terms of suction (or lack thereof) to the rest of the walls that has 1/3 PVA and then neat.
So again, all the PVA and blue grit had dried, gave everything 2 coats of multi finish and it all went on okay, no problems there. Again, everything seemed perfectly okay when drying, it wasn’t until I drilled a hole in the wall that I noticed this problem.
I’m trying to think why this has happened, the theories I can come up with so far are:
The neat PVA/blue grit have formed a film, as expected and drilling into the wall is de-laminating this and also the skim. The only problem with this is the 1st coat of skim isn’t going anywhere and as I had hoped is stuck to the old render really well.
Another idea is that, because the walls were very well sealed up, the 1st coat couldn’t dry out without pushing against the second coat, causing it to adhere poorly.
As mentioned previously, with the weather being the way it is at the moment I have thought that maybe that has in some way contributed? I’m renovating the house quite a bit and for heating at the moment I’ve just got a few plug in electric radiators. It hasn’t felt freezing inside, but again not much in the way of heating. I have done all the mixing in this old lean to thing outside, not fully outdoors but near enough and at times the weather was almost freezing, if not actually freezing. I used water from the storage tank in the loft for mixing as I thought using the outside tap would be colder and possibly more problematic. The tank in the loft and all the pipework aren’t even a year old so no chance of contamination. They’re all well insulated as well so it was the warmest (without being actually warm) source of water I had available. Having said all that, unless I’m missing something I still can’t understand why this has only happened to the second coat.
Maybe I’m just expecting too much from multi finish? It’s not as if it’s cracking or blowing anywhere and it doesn’t exactly come off without a little persuasion. But still, all it takes is one hole to be drilled and the immediate 3/4” diameter around it will crack and blow, is that normal for fresh multi finish?
Haven’t tried rolling paint on it yet and I’m a little apprehensive about doing anymore until I can figure out why this is happening.
Even where I’ve cut back some of the excess plaster from around the socketback boxes, no blowing or cracking around there.
If any of you actual plasterers can advise me on this matter I would be most appreciative. I haven’t checked if the ceiling has the same problem just yet, that was blue grit and neat PVA over old paint and then 2 coats of multi. Would that be worth checking to see if it’s havthe same problem?
On a side note, does anyone know how thin foam adhesive can set and still successfully hold plasterboard to a block wall? I ask as I’m contemplating doing this and just taping/jointing everywhere instead. I’m very much a complete amateur plasterer and although I’m happy enough with the finish I can achieve I would rather do that than carry on like this if I’m going to have to put up with this kind of a ballache everytime I try and skim!
It’s frustrating, because as I said I’ve already fitted new door linings and I fitted these so that they would be as wide as the existing walls and render, plus a little extra for a new skim coat to finish flush with them both sides.
This amounts to around 1/2” - 3/4” between the blockwork and the face of the door linings, which could make dot and dab, batten&board or foam adhesive difficult to use without bringing the plasterboard proud of the door linings.
Anyway, you have my thanks if you’ve read all this way and again, any advice on these matters I would be very appreciative of. Thanks!
*Edit:
Just tried the ceiling, same 8mm masonry drill used in the same
combi drill, not on hammer, same as the walls. Not had the same problem there although in fairness I only attempted this in one spot.
If it makes any difference, I gave the ceiling a coat of bonding before 2 coats of multi finish because it’s very wavy. Otherwise it was prepped practically the same as the walls. One coat of blue grit and one coat of neat PVA.
