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3M 4200 is a combination adhesive and sealant.

Lifecaulk is a sealant.

While some disagree, I only recommend a polysulfide sealant for the seam and for under the head of every screw.

For those with a Desert Tan gelcoat, there are several exterior siding caulks that come close to a color match.

There are various suggestions on applying a smooth bead. My personal experience is to use a small cut in the tip of the cartridge and make each side one continuous run without stopping or removing the tip from the seam.