Apply Caulk over Backer Rod

What happens here

Once the backer rod is firmly in place within the gap, take a caulk gun loaded with caulk (again, 'Tower Tech' is shown as an example). Apply a consistent bead of caulk over the installed backer rod, filling the remaining space of the gap. This technique ensures that the caulk bonds only to the two adjacent surfaces (the window frame and the siding) and not to the back of the joint. This 'two-sided adhesion' allows the caulk to expand and contract effectively with temperature changes, significantly increasing its lifespan and preventing future bulging, especially in environments with fluctuating temperatures.

Goal: Seal the gap effectively with caulk, ensuring proper flexibility and durability.