Why emulsion paint crazes over acrylic caulk?
Emulsion paint crazes over acrylic caulk. User error? Maybe. But it can happen, even if you go by the book, and is an issue that, 30 years on, is still quoted as “a new one on us” by paint manufacturers.
Here is some information on caulk, how to use it in general decorating, and for some scientific perspective, paint chemist Keith Harrison at Newlife Paints outlines why standard emulsion may be incompatible with standard acrylic caulk.
As a bit of background, caulking is an age old practice for filling joints and cracks. Black jack / bitumen is an old favourite. On wooden boats, I used a fair bit of cotton caulk to fill and seal the joints between carvel planking. Simple science.
Acrylic caulk in a tube is a wonder of modern chemistry, but relatively speaking, it has been around for years. It came out in the USA in the late 60’s and along with latex caulk, is the favoured joint-filling product for decorators. In the late 80’s this UK painter can remember transitioning over from polyfilla pushed into cracks around doors with bleeding fingers, over to a gun applied flexible filler that dried quickly, didn’t crack and was paintable. Painters Mate was a revolutionary step in UK decorating.
In 1992, I remember I saw paint crazing over acrylic caulk. In 2013, the “Oh, I never heard of that before” is still not unheard of from paint company reps.
How to use caulk
If you go by the book, you work in a room at 70 degrees F. You rake out cracks around door frames, remove dust, cut the nozzle a fraction wider than the gap you want to fill, run a thin bead of paintable acrylic caulk in the cracks, smooth off with a pointing tool (ie finger or other method). When cured, you paint over the caulk with the vinyl matt or contract matt emulsion you are using on the walls, and job done – no ugly cracks, nice sharp lines between wall and frame –
what it doesn’t say on the instructions…
you go by the book and the emulsion paint may well craze over the caulk!
30 years later, the UK paint companies, caulk manufacturers and merchants who sell caulk still haven’t communicated to end users, the compatibility issues between the normal caulk and normal wall paint available from every mainstream outlet in the UK. But if you read the take of Newlife Paint’s chemist, Keith Harrison, on what is going on, you will get a good sense of what to look out for. (Cheap will not necessarily make you cheerful.)
Paint chemist’s thoughts on caulk and crazing paint
I’ve had to think a bit about this and researched caulking materials a little.
As you mention, caulk is generally either water-based acrylic or silicone. My (limited) experience is they are both a bit of a problem, hence some manufacturers offer coloured caulking. I would hazard a guess that matt finish acrylic and vinyl may both suffer problems, but that silicone based should likely be the biggest cause of incompatibility, although they may be used less often for this sort of work.
(With decorators caulk in general) I think there are several issues:
1) Surface incompatibility (surface tensions) with water-based paint, for which the silicone is the worst culprit. It can prevent emulsion adhering totally and a generous coat of emulsion, then a second coat may hide this happening – until it cracks and crazes.
2) Flexibility – the caulk is much more flexible than emulsion, so with expansion/contraction from temperature changes again the interface between the paint and that caulk will be stressed and can cause cracking.
3) Impermeability – because the caulk itself is likely to be quite moisture impermeable, the paint over the caulk will not dry and cure as quickly as surrounding areas, and then if given a second coat, the well know “mud like cracking” can occur especially using emulsion with a high filler level. Again, a second coat “hides” what goes on below the surface.
4) the caulking used may be quite a thickness e.g. 3 – 5 mm deep. If so then various substances can continue to dry out of the caulk over a long period of time, certainly weeks or even months. These substance may mostly be water, but other substances such as coalescents (often from the VOC’s in the caulk) could continue even for months, and if so this is very likely to affect a thin coat of paint used over the surface.
In practice, then yes a primer with an eggshell, or even silk finish should make a good difference. These are polymer rich coatings (compared to matt finish), with better adhesion and also more flexibility than a matt coat. This would act as a good primer between matt paint and the caulk itself.
Where possible allow a good cure time before painting over caulking even before priming.
I asked Keith about siliconised acrylic caulk.
…siliconised may well mean they have chosen a water carried dispersion as an additive only to assist with the flexibility, without the problems of a “normal” silicone – that’s those that smell of vinegar (acetic acid) when they cure.
And I asked him for a bit more info about the Soudal Acryrub caulk, and any technical clues as to why it has worked well for me in the past. (Not the B&Q version, but the product recommended to me and supplied by UK Sealants.)
I know Soudal fairly well, and would rate them as very technically competent. They likely spent considerable time designing their caulk specifically for this purpose, hence the good result. I’ve checked their MSDS and their volatile level in Acryrub is very low – ideal for this purpose, so when using a deep fill there will be little in the product to continue to evaporate out after normal curing time, and cause problems.
The answer probably lies with using a caulk specially designed for this purpose, not a general purpose “flexible filler/caulk” that can do lots of uses, and just includes “can be overpainted” as an extra. There are lots of cheap fillers and caulks on the market, but I would say Soudal is well priced and tends to have products well designed for their end use.
Background to the question of crazing caulk
Decorators may say there was no issue back in the day and it is a problem arising since the 2010 paint reformulations when they removed VOCs from paint for the betterment of mankind. (Sounds like they should have removed VOCs from acrylic caulk first?)
1992, I can recall being phoned up to go to a new executive home and suggest why the paint had crazed around every single door frame and along the tops of all skirtings. It had been caulked with “Painters mate” and walls painted with a good trade vinyl matt emulsion. The cause of the 1/4″ wide band of crazing? We didn’t have a clue and the paint rep sure wasn’t letting on either!
Forward to 2010, the same crazing issue is still being reported by decorators. It had been a reported glitch for years, but was always “a new one on us” when you asked paint companies’ tech support.
2011 I asked Dulux tech support for a step by step on how to caulk. They said they didn’t have anything to do with the Dulux Decorator Centre branded caulk that was crazing, but they kindly stated their proposed approach – room temperature, bead size, drying time etc. It still failed.
The likely universal solution for emulsion painting straight over caulk without pre-priming it, is to use a high spec acrylic caulk that is very low in VOCs, that cures quickly and can be overpainted. These products are about, but they aren’t 20p for a dozen tubes.
Alternatively, if you are going to stick with cheap caulk, spend your time pre-priming caulk with oil base undercoat. This is an approach that seems to work every time, but makes cheap caulk very expensive in terms of labour costs.
High acrylic content emulsion seems to be more reliable than vinyl matt over acrylic caulk. (I only have anecdotal proof of that!)
A paint company tech support recommended acrylic eggshell, which Keith referred to and explained the technical reasoning as to why it would likely be a compatible primer. Acrylic eggshell will dry a lot quicker than oil based undercoat too.
An alternative is to use flexible filler like Toupret’s Elafib. Or the scrim tape and overfill. Not as convenient as a bead of caulk.
I am sure there are some brand product suggestions out there to help advance / minimise this issue. Any thoughts welcome, especially from painters in the USA. What do you use, is this a problem for you?
For an overview, This Old House has an easy read article on all things caulk.
And this is The Environmentally Responsible Construction and Renovation Handbook from Canada with more info on caulks and their use in a home environment
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