Over the last few years, the cracking of caulk has been a popular subject at The Decorators Forum and decorators are still on the lookout for a caulk that doesn’t crack. We figured We would share our findings after getting to the bottom of this issue.

What causes the paint cracking?

A lot of painters think that it’s down to the caulk they use, but it’s not just down to the caulk itself in fact, but the paint plays a major role in the problem!

They have lost a lot of their elasticity and dry even more brittle in contract/vinyl matts due to the newer chemical composition of paints. This is why it was a very unusual issue 10 years ago. Caulk has always had anywhere from a 4-15% shrinkage rate during drying, and over the years the general formula in caulks has not changed much, but the paint has.

Drying times: One of the reasons is that the caulk isn’t 100 percent dry. Even if the surface may appear dry, it may still have some shrinking to do, even if left overnight. Therefore, the paint film dries when painted over it, and then shrinks immediately after the caulk begins, tearing the paint film apart and leaving cracks on the surface. This is why when using soft sheen/vinyl silk/masonry & glosses, etc., you do not have a problem as they are versatile, and they shift with the caulk and do not break.

Temperature

Another explanation for paint cracking with emulsion-based paints may be attributed to the application of the paint on a too low temperature substrate. For good film growth, emulsion based paints need a minimum temperature. This temperature is normally around +7 ° C. If you use emulsion paint to paint during winter, the surface temperature of the sealant can be too low. Therefore, even though the inside air temperature is high enough (above +7 ° C), the sealant’s surface temperature may be too low and the caulk may take longer to dry fully (even if it seemed touch dry). This will result in the paint-film cracking.

Thicker application: The thicker the bead, the longer it will dry completely and the more it will shrink back, with the shrinkage rate of caulk and drying times, even though the surface appears dry.

Newly branded caulk

As cracking is becoming a frustratingly common problem, many decorators are on the lookout for a new caulk. This has made it used by producers as a selling point.

These ‘anti crack’ slogans written by all of these manufacturers on new goods are all well and good… until you read the small print. You note a little about the “limitations” of the item on a lot of different “anti-crack” caulks on every data sheet we have looked at. They all say things like, 

“Some paints have very little movement in the dry film (especially certain vinyl matts) that could cause cracking”

“Painting over with highly filled water-based paints in the paint film can cause cracks”

“Highly filled emulsion paint overpainting can cause cracks in the surface of the paint.”

There are plenty of other ways of saying the same thing around it. The “highly filled paints” they refer to are almost all the contract/vinyl matts you get for all the new and improved formulations on the market.

How to solve this problem

So, after browsing through all the data we could find on caulks, newer paints and a good year trying different caulks with different paints in different conditions, this is our suggestion:

As an aerosol, you can buy Coverstain and it is not only a cheap remedy, but it is quick and simple to use. All you need to do is go round and gently spray your caulk beads, allow them to dry, then paint over as usual.

Another strategy is first finish the woodwork, then tape off the woodwork (or some would say trim) and then finish the emulsion. This means that you cover the caulk while coating the woodwork with gloss/satin, etc, providing a layer that stops the emulsion from cracking. Some decorators frown on it because you need to use tape, but in a few empty rental properties that we decorated with brush and roller, we tested this theory. we were surprised at how it was speeding me up. It left crisp straight lines with no effort (that is more than capable of doing by hand). Being able to whop on the skirting boards without having to think about getting a line, and the same thing really speed us up with the emulsion, and…. There was no madness.

We hope this has helped.