Why does Paint Crack when you paint over sealant?

There it is. Your beautiful siding job marred by paint cracking. It’s not cracking everywhere – just on top of the sealant. Have you ever been there? Or have you painted over caulking in an interior application such as crown molding where the sealant seemed to last, but the paint cracked? You’re not alone, and today we’ll take a look at what causes paint to crack over sealant as well as some tips and tricks to avoid it.

What’s The Deal?

It’s important to understand what’s happening when paint cracks over a sealant. It’s probably being caused by one of these things.  Either…

1. The paint was put on too soon or
2. The paint isn’t flexible enough
3. The sealant bead is too small
4. The sealant is the wrong technology

To be clear, in both of these cases, we’re referring to a situation where the sealant hasn’t cracked…the paint has.  If you have cracked sealant, there are different things going on.  Let’s break down both scenarios.

Scenario #1 – Painted Too Soon

Most caulking shrinks as it dries.  There is water (or solvent) inside the caulking that leaves the film.  When this happens, the film naturally shrinks.  Because the bead is larger in the middle and thinner on the sides in a traditional joint, it shrinks the most in the middle to yield a concave appearance.  The sealant is designed to do this.  It’s been formulated to dry even when there’s a thick bead.  Paint isn’t.  Paint is designed to dry quickly.  If you put paint overtop of a sealant that is still shrinking, it automatically forces the paint to flex.  A high-quality paint might stand a chance, but a lot of them will fail within a couple days.  That’s actually how you can usually tell that this was the problem.  If the paint cracks in the first week, it’s probably from painting before the sealant was fully dry.

Scenario #2 – Paint Isn’t Flexible Enough

When a sealant is functioning properly, it’s moving.  A lot of paints just can’t move with the sealant.  This is especially true of the “highly filled” ones like primers and flat sheens.  The flatter a paint, the more likely it is to crack when applied over a sealant.  If nothing else works, try upping your paint spec to a higher quality or higher gloss paint.  It may just solve the issue.

Scenario #3 – The Sealant Bead Is Too Small

This can be related to “scenario #2.”   The smaller the bead of sealant is, the higher the percentage of movement that you’re asking for when it moves.  For example, pretend that our siding moves ¼” every day.  If our bead is ¼”, then the bead has to double in length to put up with the movement of going from ¼” to ½” and back.  It’s moving 100% of it’s original length.  Now imagine the same joint except we have 1/8” bead of sealant.  Instead of moving 100% every day, it has to move 200% every day.  The sealant is probably going to be ok in both cases…but the paint on top of the sealant is being asked to do the same thing.  A bigger bead of sealant will make the sealant more durable…but it will also make the paint less likely to crack. 

Scenario #4 – The Sealant Is The Wrong Technology

This one is pretty easy to diagnose.  Silicones are not paintable.  The failure won’t look like the images above…it’ll look more like this. Silicones are pretty easy to identify – they say “silicone” on the label.  If you find yourself in this situation, you’re in for a bit of a rough redo.  The process of removing a silicone is one we’ve covered before.  You can find it here.

So to ensure success the next time you are planning to paint your sealant, ensure that you use a fairly large bead, give the bead plenty of time to dry, and coat it with a high-quality paint.  You also have an alternative that could help prevent the issue in the first place.  eXact Color is designed to give you a perfectly-tinted color of sealant that you won’t have to paint.  You add the paint to the sealant instead of having to wait/come back to the job to paint it.  To learn more about eXact color, head here.