Caulking Deep Cracks
To caulk a crack, all you do is fill it, right? Yes, but not if the crack is a deep one. You need to do something else first in this case. It’s OK to use high-quality caulk on cracks up to about 1/8-inch wide and 1/2-inch deep, but caulk alone should never be used to fill larger gaps.
Deep or wide crevices should be lined with foam backer rod first. Use a putty knife to force the flexible backer into the crack until it sits at least 1/4 inch below the surface. Then fill with caulk.
Backer rod will help support the bead and prevents caulk from sinking down as it ages and dries. This technique is especially useful for windows, doors and brick walls. Foam backer rod is sold in various diameters and lengths at hardware stores.