Backer Rod
If the joint or crack to be sealed is 1/4" wide or wider, it is best to install foam backer rod in the joint - to the proper depth - before applying the caulk.
The reasons are:
1) It saves money. Backer rod is generally cheaper than a good quality caulking compound, and most of the joint can be filled with the backer rod before the actual sealant is installed.
2) It provides the means to form an "hourglass" cross-sectional shape to the bead of sealant. This geometric shape allows the sealant to handle the inevitable joint movement much more easily than any other configuration. (The reason: Large surface areas of adhesion are established at the sides of the joint, while a relatively thin cross-section of sealant is left in the center of the joint to allow for easy flexing)
3) It provides a "bond-breaking" surface at the rear of the joint or crack that prevents the sealant from establishing three-point adhesion.
4) It allows for additional pressure to be applied to the sealant during the "tooling" process, which further favors more sealant being forced into intimate contact with the sides of the joint for better adhesion.
Note: Backer Rod is easily installed in joints by simply pressing the material into the recess to a depth that will allow the thickness of the sealant to be approximately 1/2 that of the width of the joint. It can be installed just by pressing it into place with a finger or an appropriately sized tool. If you are using closed-cell backer rod, it is important to avoid damaging the surface of the rod (such as with holes or nicks) because wherever such damage occurs there is a risk of "out-gassing" from the backer rod and forming blisters in the sealant.
Note 2: Open-cell backer rod does not have this problem, but it has a much greater tendency to absorb water and hold it - with potentially very negative effects on the sealant - if the integrity of the sealant is breached at a later date. Then, masking tape can be applied to both sides of the joint before caulking to prevent smearing on the exposed surfaces adjacent to the joint.
You should select backer rod based upon your particular needs. Open-cell type is recommended to speed drying. A closed-cell type is more moisture resistant.
Information in this document is subject to change without notice. Other products and companies referred to herein are trademarks or registered trademarks of their respective companies or mark holders. Please Read our Privacy Statement.
Backer Rod 
