Exterior Caulk Joints to Battle Exterior Conditions

Exterior Joint Caulk

Folks often ask PennCoat about the best way to seal exterior joints to keep them airtight and watertight. You have several alternatives, but one of our favorite caulking compounds is a polyurethane elastomeric sealant. It works well on fixed and working joints at least 1/4-inch deep. It can be applied in horizontal and vertical applications, adheres to just about anything, and even works on underwater joints. As you might remember from a previous blog, polyurethane is a polymer of NH-(C=O)-O monomers. Polyurethane elastomeric sealnts are often 2 components, one part of polyisocyanate prepolymer and one part xylene to form the sealant.


Polyurethane elastomeric caulk adheres strongly to concrete, masonry, insulation board and other exterior joints. The mixture has an aromatic odor and a packing density of 12.95 lbs./gallon. The volatile organic compound for the material PennCoat commonly uses has  19 g/l for Part A, 92g/l for Part B and 38.1 g/l for the mixture, a VOC content less than 4 percent. The tear strength is 45 lbs./inch and the tensile strength is 120 psi. It has excellent weathering resistance and good resistance to water, dilute acids and alkalines, and residential sewage.

The caulk resists dirt, staining, discoloration and delamination. Because the sealant is completely unaffected by water action, it is perfect for sealing cracks and joints in underwater applications, such as reservoirs. It cures chemically and does not sag. It’s approved for exterior insulation and finish systems, including systems with drainage.

Preparation and Application

Surfaces must be sound, clean and frost-free. Remove any oil, grease, residues and foreign matter before applying. If you want to prevent the caulk from bonding to the bottom of a joint, apply bond breaker tape or backer rod first. You need not prime the substrate unless it will be submerged after curing. Some manufacturers of exterior insulation and finish systems recommend priming.

To mix, pour Part B into a pail of Part A and then add the contents of the Color-pak, if desired. Mix with drill paddle running at low speed (400-600 rpm) for three to five minutes, periodically scraping down the sides. In colder weather (below 50°F), mix the top half of the pail for a minute before plunging deeper. Don’t apply when temperatures drop below 40°F or rises above 100°F. It’s best to apply the caulk when the joint is in its daily midpoint between maximum expansion and contraction. Refrain from caulking frosted joints. Apply with a caulk gun or a follower plate loading system. Place nozzle at joint bottom and completely fill with a steady flow. Apply in one continuous path rather than overlapping, which traps air. The ideal joint is two times as wide as it is deep.


Ensure joint is at least 1/4 inch deep and that maximum daily variation of joint width doesn’t exceed 50 percent. Prevent contact with silicone residue, alcohol, chlorine and solvents before applying and during cure. Curing time is six to eight hours. For water-immersion applications, allow three days curing time. You might want to apply a UV protectant to light-colored caulk.

Part A can irritate skin. Part B, which contains xylene, is combustible and can irritate skin and our respiratory system. When mixing, avoid contact, wear gloves, goggle and respirator. You can clean up uncured caulk with an approved solvent. Once it cures, the caulk can only be removed mechanically.