The ultimate caulk/paint compatibility test

Now, if you are reading this, it’s most probably because you have had a problem with caulk. Whether it’s cracked or discoloured, it’s frustrating. That is why I have researched and written this blog, outlining the ultimate caulk compatibility test I carried out. A lot of people presume the problem lies with the caulk alone, but it’s actually the paint that is the problem.

It’s all down to the chemical makeup of the paint that is applied over the caulk. Hopefully this compatibility test will help you find the best decorators’ caulk to use.

Why is caulk cracking?

Caulk absorbs moisture from the paint, making it expand. The paint film dries first, then as the caulk dries it contracts, pulling the paint film apart. Another factor may be the caulk not being totally dry; the same thing happens again.

.

Why has caulk changed the colour of paint?

Almost 95% of all the water-based acrylic sealants on the market contain plasticizers, and these plasticizers can migrate into the paint, discolouring it. Depending on the binder and filler type of paint, it may or may not happen.

The Caulk Compatibility Test

I decided to test “off-the-shelf” trade vinyl matt products, all tinted similarly to the Dulux colour ‘Polished Pebble’, although some came out slightly different. I put these against every caulk I could pick up from each merchant. I conducted the test on an MDF board primed white. I applied each caulk in even beads vertically, smoothed it out and left it for 24 hours to dry. I then put a horizontal line of each paint crossing each caulk, applying two coats with six hours between coats. You would think that a material such as caulk would hold up when painted without any issues. After all, that’s what it’s designed to do!! Let’s see how I got on…

The results

These where completely different to what I was expecting…

See for yourself.

So, it turns out the best “off-the-shelf” caulk for not cracking with other paints is Brewers Ultimate, Followed second by Fat Hog Standard; however, that one did discolour.

The best paint was Albany, which only very slightly cracked on some caulks. I found that caulks that discoloured, did so with every paint. One thing to note: the caulk lines were thick and smoothed out on a flat board, rather than an internal corner. So, this test may have exaggerated the pull on the paint when contracting due to bigger surface tension. Meaning the ones that “slightly cracked”, you may get away with on a small bead. I have done a video below to show you the results.

I hope it will help you find the best decorators’ caulk to use on your own jobs.  If you found it useful, why not check out the Decorators Forum UK for more great content.