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Painting attention to detail with joints

Listed under Blog, Painting, products Posted Jan 06 2013

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
acrylic caulkIf 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 »

Rant about the acrylic caulk scandal!
If I used cheap / DIY emulsion, which contains a high proportion of filler (chalk, for want of a better word) and DIY caulk which is possibly a bit short on acrylic, I would understand why caulk can craze. But I use good trade (vinyl matt) emulsion, and I spend a bit more than the average on good trade caulk, and still, I, and loads of other decorators doing everything by the book, cannot guarantee the final result.

All I can say is that there must be more to compatibility between caulk and emulsion than “good” ingredients, but who knows? The manufacturers are no help. While they all insist that we use their company undercoat with their company gloss to ensure perfect results, they won’t commit to which emulsion X will work with caulk Y.

Personally, I don’t think the likes of Dulux are interested in committing, because unlike their paint which they control like hawks, they don’t have a guaranteed consistent high quality supply of caulk that they will stand by. (Caulk is a generic commodity, a bit like White Spirit, that anyone can buy in bulk from various 3rd party manufacturers, and re-brand as your own. So who knows what rubbish they will use if they can make a few extra bucks profit.) So, either I buy 5000 tubes of caulk from a supplier who will guarantee the formula works with the emulsion I use, or I rely on the undercoat trick to try overcome a problem of the manufacturer’s making.

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.

hand-painted country style kitchen

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 on the forum



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7 comments to “Painting attention to detail with joints”

  1. chris spence

    Found his very interesting.
    Why ?
    I had acryilic caulk graze and crack after overpainting with diamond matt.
    I even left caulk 18 hours before paint application.
    It was so bad I had to use Repair Care MP, which when funds are available I will caulk with completely, as I have never had a failure with it.
    I am however in agreement with your theory that cracking and grazing is a result of inferior paints.
    Little Greene having been one of very few I have never had problems with such as cracking or grazing.
    I use dow Corning for first choice caulk.

  2. Andy Crichton Andy Crichton

    Thanks for that Chris, sorry too!

    Dulux do say that acrylic eggshell will seal caulk, almost an admittance that acrylic will do what non-acrylic wall paints can’t be relied on to do.

    Can anyone confirm a drama free life with Leyland vinyl matt over caulk? Supposed to be one of the better off the shelf trade emulsions. If my theory is correct though, any vinyl matt emulsion is likely to fail as not.

  3. daron

    Can just add that I have finished emulsioning my entire newly plastered house a few months back with 1 wash coat then up to 3 coats of Leyland acrylic vinyl matt brilliant white emulsion. Have encountered hardly any cracks. As good or better than than Dulux brilliant white (a bit yellowy) and much better than Wickes stuff. Can’t afford Little Greene.
    Best regards
    Regards

  4. Andy Crichton Andy Crichton

    Hi Daron, I agree with everything you say except about not being able to afford Little Greene. On new plaster, dilute first coat of acrylic matt 15% and apply second coat (same colour), I reckon your work will be done. Substitute second coat with Intelligent Matt, and it will be tough. Use a 3/8″ Wooster pro-doo z, job done.

  5. Daron

    Correction sorry. I have used Leyland Super Leytex brilliant white matt not acrylic vinyl as mentioned (although I think it is the same thing/just another name for it?).
    I only did 4 coats when not happy with finish. Trouble is I only found your site towards the end of my work when I was getting fed up with non perfect finishes. I know now though, great site – thanks Andy.

  6. Andy Crichton Andy Crichton

    its a constant learning curve!

  7. Nad

    Hi, in regards to the Little Green Paints, I presume either of the two matt emulsions are fine for going straight over, say a Soudal Acryrub caulk then?

    From a refurbishment point of view, is it still more economical to prime with a Dulux Eggshell then go over with a trade matt emulsion? Or are there still crazing issues here?

    Thanks

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