Not sure if this is too late for you but any quality latex-based sealant will be sufficient for this application. The more important component would be the type of paint that is applied over the top, as this would be the primary protection from moisture and UV. DAP ALEX Plus Acrylic Latex caulk with silicone to name one. It's just over $2 a tube. You could buy more expensive sealants, like Vulkem, but it will end up costing you a fortune unnecessarily at $6 a tube. The paint is what keeps the caulk from shrinking/drying/cracking so make sure you use a quality paint.