Paint cracking over caulk solutions

Paint cracks over sealant – why? And how to solve it …

Sealant cracking has been a common topic at The Decorators Forum UK over the past few years and decorators are always looking for a sealant that won’t crack. After a hell of a lot of research and some personal experimentation, I thought I’d share my findings after getting to the bottom of this problem. You may also want to read my compatibility test which will help you choose which brands you use.

What causes paint cracks?

Many painters assume it is due to the sealant they are using, but in fact, it’s not just the sealant itself, the color plays a big part in the problem!

Due to the newer chemical composition of paints, they have lost a lot of their elasticity and dry much more brittle in contract / vinyl mats. So it was a very rare problem 10 years ago. The caulking has always had a shrinkage rate of 4-15% when dry, and the general formula for caulking has not changed much over the years, but the color has changed.

Drying times: One of the causes is that the sealant is not 100% dry. Even if the surface appears to be dry, it can still shrink a little despite staying overnight. Therefore, the paint film dries when painted over and shortly afterwards the sealing ring continues to shrink, pulling the paint film apart and leaving cracks on the surface. This is why you have no problem using soft gloss / vinyl silk / masonry & gloss etc as these are flexible, move with the waterproofing, and won’t splinter.


Another reason for paint cracking with emulsion-based paints can be that the paint is applied to a substrate at too low a temperature. Emulsion-based inks require a minimum temperature for good film development. This temperature is usually around + 7 ° C. If you paint with an emulsion paint in winter, the surface temperature of the sealant may be too low. Even if the indoor air temperature is high enough (above + 7 ° C), the surface temperature of the sealant can be too low and cause it to take longer to seal to completely dry (even if it appears dry). This would lead to the lacquer film tearing.

Thicker application: the thicker the bead, the longer it will dry completely and the more it will shrink back, even if the surface appears dry.

Newly branded sealant …

Many decorators are looking for a new sealant because cracking becomes a frustratingly common problem. This has led manufacturers to use it as a selling point.

Those “anti-crack” slogans written on new products by all of these manufacturers are all well and good … until you read the fine print. On every data sheet I’ve looked at, on many different “anti-crack” seals, you note a little about the “limitations” of the product. They all say things like

“Some paints (especially certain vinyl mats) have very little movement in the dry film, which can lead to cracks.”

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

“Painting over with highly filled emulsion paint can lead to cracks in the paint surface.”

There are many other ways of saying the same thing. “High Fill Colors” they refer to is pretty much any contract / vinyl mat you bring to market with all of the new and improved formulas.

How can you solve this problem?

After browsing through all of the information I could find about gaskets, newer colors, and a good year trying different gaskets in different conditions with different colors, this is my advice:

Use a thinner bead and let it dry longer. This is not always possible (especially after some carpenters) so I’ve tried priming the sealant with something that will act as a barrier (gardz, BIN, Coverstain) or I’ve managed to use acrylic primer after a few successful tests) before using the emulsion. This is quite a pain, but still a solution.

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