The cracking of caulk has been a common topic on The Decorators Forum UK over the last few years, and decorators are always on the hunt for a caulk that ‘doesn’t crack’. After a hell of a lot of research and some personal experiments, I thought I would share my findings after getting to the bottom of this problem. You may also want to check out my Compatibility Test, which will help when choosing the brands of caulk use.
What causes the paint cracking Over Caulk?
A lot of painters presume that the cracking is down to the caulk they are using, but in actual fact, it’s not just down to the caulk itself, but paint plays a big role in the problem!
Because of the newer chemical makeup of paints, some have lost a lot of their elasticity. Contract and vinyl matt emulsion are particularly brittle and cracking over caulk when using this type of paint is common. Caulk has always had anything from a 4-15% shrinkage rate while drying and the general formula in caulks haven’t changed much over the years, yet the paint has.
One thing which can amplify the problem is caulk not being fully dry before you paint over it with emulsion. Even though the surface may seem dry, even if it has been left overnight, it may still have some shrinking to do. Therefore, when painting over it, the paint film dries and then shortly after, the caulk continues shrinking, pulling the paint film apart and leaving cracks on the surface. This is why you don’t have a problem when using soft sheen/vinyl silk/masonry & glosses etc, as they are flexible and they move with the caulk, so doesn’t split.
Another reason for paint cracking with emulsion-based paints can be down to the paint being applied on a substrate with a too low a temperature. Emulsion based paints need a minimum temperature for good film development. This temperature is approximately +7°C. If you are painting during winter with an emulsion paint, the surface temperature of the sealant may be too low. So, even if the inside air-temperature is high enough (above +7°C) the surface temperature of the sealant could be too low and result the caulk taking longer to completely dry (even if it seemed touch dry). This would result in the cracking of the paint-film.
With the shrinkage rate of caulk and drying times, the thicker the bead of caulk, the longer it is going to fully dry and the more it’s going to shrink back. So, if you’re searching for a solution to paint cracking over caulk, you may want to start by applying thinner beads of caulk.
Some Caulk is More Prone to Cracking
A lot of decorators are on the hunt for a new caulk as cracking is becoming a frustratingly common problem. This has prompted the manufacturers to use it as a selling point.
These ‘anti crack’ slogans written on new products by all these manufacturers are all well and good… until you read the small print. On every datasheet I have looked at, on a lot of different “anti-crack” caulks you notice a bit about the ‘limitations’ of the product. They all say things like;
“Some paints (especially certain vinyl matts) have very little movement in the dry film which could cause cracking”
“Painting over with highly filled water based paints can cause cracks in the paint film”
“Painting over with highly filled emulsion paint can cause cracks in the paint surface.”
There are lots of other round about ways of saying the same thing. “highly filled paints” they are referring to are pretty much all the contract/vinyl matts you get on the market with modern formulas.
How to solve this problem?
So, after scrolling through all the information I could find on caulk, newer make up of paints and a good year trying different caulks in different conditions with different paints, this is my advice:
Use a thinner bead of caulk and leave it to dry longer. This is not always possible (especially following some carpenters). I have also tried and tested ‘priming’ the caulk with anything that would act as a barrier (gardz, Primer Sealer Stain Block, Cover Stain, or I have been successful in using acrylic primer after a few tests) prior to using emulsion. This is quite a pain but still a solution.
If you have already experienced the issue and now want to save it, I’d suggest applying a coat of Zinsser Peel Stop over your cracked paintwork. This will sort the issue and allow you to repaint your caulk. Available online here.
Some caulks are less susceptible to cracking and crazing than others. The best I’ve found is Dunlop Flexible Acrylic Filler, which has never let me down. You can apply a medium sized bead, leave it an hour at room temperature, then paint over without issue.
Another method which I discovered while spraying, is finish the woodwork first and paint onto the walls (thus covering all the caulk with undercoat, gloss, satin etc.) Then tape off the woodwork (or some may say trim) and complete the emulsion after. This way you create a barrier between emulsion and caulk, preventing the emulsion from cracking.
This is frowned upon by some decorators because you need to use tape, but I tested this theory in a couple empty rental properties I was decorating with brush and roller. I was shocked at how much it sped me up. It left crisp straight lines (that I am more than capable of doing by hand) with no effort. Being able to whop in the skirting boards without caring about getting a line, and same for the emulsion really sped me up, and…. there was no crazing.
I hope this has helped.
Will emulsion paint over caulk?
Yes, providing you use a good quality caulk and a trade vinyl or durable emulsion, you will be fine painting straight over caulk. If in doubt, you can always prime caulk with Zinsser Cover Stain first. This will avoid problems with crazing.
How do you fix cracked caulking?
If it’s the caulk that’s cracked, your best course of action is to remove it and then re-caulk. If your paint is cracking over caulk, then you should prime the cracked paint with Zinsser Peel Stop and paint again.
What happens if you don’t paint over caulk?
Caulk is designed to be painted. If you don’t paint it, then it may discolour over time, or dirt might stick to it, but it’ll be structurally sound. There are several acrylic sealants that can either be left or painted. I’d suggest opting for one of these.
What do Other Decorators Think?
Paint cracking over caulk never used to be an issue. Manufacturers have been playing around with formulas to try and save money and stay competitive, which is a real pain. It makes our lives as decorators more difficult. We’re now faffing around with different products to try and find a combination that works.
I’ve seen loads of people complaining about paint cracking on caulk, but I’ve never had an issue. Just don’t lather it on and leave it 24 hours before painting. If you’re caulking woodwork up, why not undercoat before painting your walls? It might avoid having problems.