Painting attention to detail with joints
There is a big difference between a professional job and a DIY one: sharp lines v wobbly ones, smooth surfaces v lumps and bits… I try to take a good professional job another level higher, by paying more attention to detail than the last decorator.
Hairline cracks in joints
If you look up at the joint where the ceiling and wall meet, you may see a hairline crack. Basically, this is a central heating issue! When the room heats up and cools down the plaster and the board beneath the plaster expand and contract at different rates, ending up in a crack at the weakest point.
To deal with these movement cracks, decorators use a flexible acrylic “caulk” . This product fills the joint and moves more than the materials it is bridging. It isn’t fool proof in this day and age of hot houses, but it is the recognised decorating trade solution to the hair crack problem.
Caulk is ideal for an invisible quick repair, but it is tricky to apply because it skins over quickly. It is OK if you get a perfect result first or second time, but if you keep fiddling with it with a dry finger, it beads up and you end up with snots on the ceiling and walls which you cannot sand down easily, or at all. Paying attention to detail, you should keep a wet rag or a wet wipe with you at all times to clean up.
Also, if you rely on your finger to get into corners, you lose the sharp 90 degree angle. A simple solution is to use a screw driver and damp rag to form the sharp right angle.
Update Soudal Acryrub is a very flexible acrylic caulk that expands and contracts up to 9%. It can also be lightly rubbed over when dry to remove any undesirable snots. I love this stuff, it doesn’t appear to let go in normal use around tops of skirtings etc. Just to push it, I have caulked a wide gap down the side of garden gate post, holding a heavy gate that slams and jams. No way should it be able to hang on, but a month has gone by and it is still fine. So I will report back in a few months.
Hairline crazing on joints
Sometimes, acrylic caulk solves the issue of cracking, just to create another issue on joints – hair line crazing! You may even see it, if you take a really close look at where the ceiling and wall lines meet. When you apply flexible acrylic caulk and then paint over it with vinyl (matt) emulsion, they dry out at different rates, and the difference in surface tension can create a subtle crazing effect in the paint along the line of the joint.
Update
I used to think I was covering my bases by using high quality vinyl matt emulsion and mid-range rather than cheapo trade caulk. I also tried to let caulk harden overnight (even though 2-4 hours is the recommended time) and for belt and braces, I painted a coat of oil based undercoat on top of the acrylic caulk before emulsioning it.
This arcane approach seemed to cut out that particular crazing problem, most of the time – but only most of the time! I got let down occasionally, even bending over backwards to get it right, and there seemed to be no rhyme or reason to it.
I believe I was barking up the wrong tree with the rant below. Basically I now think the issue is not about the caulk, but standard vinyl emulsion- it is low in acrylic resin, high in vinyl polymer content and is therefore not compatible with acrylic caulk.
Conversely, the high acrylic content premium matt paints I use exclusively now, don’t cause any crazing whatsoever with caulk- Little Greene or Mythic. And the Dulux Tech support advice, finally, was to use Diamond eggshell as a sealer coat over acrylic caulk. It pretty much worked. Guess what, Diamond eggshell is a premium product, probably high in acrylic resin content, and low in vinyl content, so it tends not to react with acrylic caulk. Could it be so simple? The bottom line, if you don’t want to bend over backwards and still risk failure with caulk, use high quality acrylic emulsion, not cheapo vinyl matt with any half decent caulk from one of the paltry 3 manufacturers supplying our dec centres these days.
Rant about the acrylic caulk scandal »
So really, acrylic caulk is not the weakpoint in the crazing debate, vinyl matt probably is. And that is why they have probably kept schtum, because in truth, their “premium” emulsions aren’t that high quality compared to the “posh” companies favouring high acrylic resin content.
Bombproof solution to cracked joints There are no cracks along the joints in the ceiling in this kitchen I decorated, because, at the construction stage, I reinforced all the joints with thick paper tape and overfilled with dry lining joint compound.
joints taped and overfilled with dry lining compound
The theory was, that unless the whole house moves on its foundation, the tape won’t break under the effects of normal heat movement. And under normal conditions the joint compound can flex better than normal gypsum plaster. If the tape is solid and the filler flexes, the joints are unlikely to crack. The practice proved the theory right. I decorated this 12 years ago and it is as good as new.
Update
Another 2 years on and the joints are still as solid as the day they were sorted out. I think another reason for their longevity is that it is a single storey building, so apart from occasional scurries into the attic, there is no regular weighty movement to deal with.
Control the thermostat
Another crack reduction option in modern homes is to keep a constant temperature. Gypsum plaster seems to crack if you look at it funny, and doesn’t do well with the walls and ceilings shrinking and expanding daily.
Controlling the thermostat is a bit beyond the brief of a decorator, but we try to do our best.
If you have any other tricks and tips about hairline cracks and acrylic, pleases share them in comments. And if you have a caulk related problem you cannot solve, ask the expert.
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