So you filled that wide gap in the exterior trim with the best caulk you could buy, and the next year it had pulled away from one surface, leaving a large gap. Or you tried to fill a wider gap, and the caulk just fell in the hole. What went wrong? No backer rod.
Before professionals fill a large gap with caulk, they bridge the wide opening with a stiff foam backer rod. The backer rod is wide enough so friction holds it just below the gap’s surface. The rod supports the caulk applied in an hourglass shape with a height-to-width ratio of about 1:2.
Why? Caulk needs to expand and contract as surfaces move. The hourglass shape allows the caulk to bond to only two surfaces; the narrower section quickly expands and contracts with movement. Caulk should never fill a space. It should never be applied to three sides or an unbridgeable wide gap, or it will quickly fail. Caulk can’t expand and contract when it is pulled in three directions or when the cross-section is too thick.
You will find backer rods in larger paint and hardware stores. It is sold in lengths like rope, and it comes in various diameters. Choose a diameter that is wider than the gap to be filled, and force the rod into place with a blunt tool or putty knife.