How to Caulk Concrete Control Joints

March 9, 2020

The keys to a clean polyurethane control joint between two concrete slabs 

Before squeezing sticky stuff into the gap, get a grip on the process.  Preparation steps before sealing a control joint in a concrete slab:

1: Use a polyurethane caulk, NOT the self-leveling kind. Matt uses Tremco's Dymonic Fast Cure, but Sikka makes one, as does NP1.

2. Don't get a cheap caulk gun. Just say no to cheap tools.

3. Rather than tubes and tiny guns, choose a sausage system. Foil sausage tubes provide more product, less waste, and less packaging than plastic tubes. They also allow you to change the tip size on the caulk gun. 

TIP: When cutting the end off of the sausage tube, use side-cutters, not a utility knife. Avoid at all costs the opportunity for any of that polyurethane caulk to get on any part of you.

Because a tool is an extension of a carpenter, the utility knife applies. Side-cutters allow you to snip the end off cleanly in a single motion, so none of the polyurethane is pushed out of the end, nor does it stowaway on the tool.

4. Other tool/products needed for the job:

  • Knee pads. They help preserve the pants and the knees.
  • Masking tape. Multiple widths, from a quality manufacturer. Matt chooses SCOTCH masking tape from 3M.
  • 409 Multisurface cleaner
  • Fine silica sand. It is the key to a perfect job that will last (and look good) for a long time. Fine silica sand is better than play sand, which tends to have little pebbles and rocks in it. Matt uses plastic squeeze bottles or larger juice containers to dispense the sand, and a small paintbrush to distribute it.
  • Paper towels for cleaning polyurethane from the nozzle.

With the pile of tools defined, Matt heads to the job. Clean the joint with a diamond wheel on an angle grinder to make a uniform gap between the driveway and garage slab. After cleaning it up, clean it out with a leaf-blower or compressed air.  

Wear PPE when grinding concrete. Silicosis is real.


The tape defines the line of the concave bead

Apply masking tape to both sides of the joint just below the crown of the tooled edges. The convex edges will ease into the concave joint for a smooth appearance.

Apply the tape to both sides with finger-pressure to seal the edge of the tape and to define straight lines. The joint will be concave, and the edges will be able to feather out into the sides without encroaching on the driveway.

At T-intersections, miter the tap, overlapping the miters, to yield a sharp corner.


Work in short sections—depending on the weather

Matt moves from right to left because he is right-handed. Your mileage may vary. Don't dispense more than you can tool at a time. If the wind kicks up a little bit, it can blow dust, leaves, and other debris into the wet polyurethane, which will ruin the appearance of the finished product.

Try to apply the caulk to be as close to how the finished product should look as possible. Look for the slight concave finish with the joint full. Don't starve or overfill the joint.

Let the caulk work its way down into the joint. Let the bead build ahead of the tip, to you can see that it is getting down to the bottom of the crack.

Hold the gun perpendicular to the control joint, at about a 45-degree angle, squeeze the sausage, and slide sideways. Allow the polyurethane caulk to push its way ahead of the nozzle so that you can see the caulk fill the joint.

The rounded trailing surface of the nozzle will form the rough concave edge on the bead. The idea is not to get the finished bead while applying, but the closer you can get, the easier it is to make it look great later.


409 doesn't seem controversial, but watch this

The instructions on the Tremco Dymonic tube say to dry-tool the joint. However, other Tremco polyurethane products say right on the container that mild soapy water can be used. 

Some people think that using 409 in this way will cause the polyurethane caulk to bubble, but Matt says that he has done thousands of feet of control joint this way and has never seen bubbles, failure, or even mild problems. He also offers this alternative:

"If you've had conflicting experience, or just don't like this idea, well, then just don't use it."

Dry-tooling polyurethane is difficult at best. It is sticky and difficult to work with. A quick spritz of the 409 and a fingertip makes that polyurethane tool like magic.

The critical detail is feathering the bead at the edge of the masking tape rather than tooling in a step.


Finish the bead

Remove the masking tape across the bead, so that any sealant that may have built up along the edge of the tape is directed back into the joint. The side can be cleaned up with a spritz of 409 and a finger

Generously sprinkle silica sand onto the tooled bead. The excess will blow away after the polyurethane has dried. The protective sand coating does two things:

It protects the wet polyurethane caulk from bugs, dirt, and debris (it also protects shoes who have unconscious owners).

It provides a rough surface finished texture similar to freshly finished concrete.


—Matt Jackson is a master carpenter, remodeler, SketchUp Wiz, YouTuber, and contributing editor to ProTradeCraft. He lives and works in Rapid City, South Dakota. 


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