I would use an exterior oil based primer.
A Primer is a coating that adheres well to the substrate and improves the adhesion of the subsequent top coat.
A Sealer is a coating that prevents fluid flow into or out of the substrate.
EVERY oil based or latex primer contains huge rocks that are almost large enough to see with the naked eye. It's those rocks that both make the primer dry to a rough surface which allows the subsequent top coat to stick better, and also it's those same rocks that try to flow into the porous surface of drywall, concrete, joint compound and even wood to plug up the surface porosity and therefore seal that surface so that most of the primer remains on the surface rather than being absorbed into it like pure clean water or mineral spirits would be.
So, every primer you're likely to see in a paint store will also be a sealer because them same huge rocks do both jobs. It's just that when you're painting a surface like vinyl siding or metal (or even glass), your Primer/Sealer will only work as a primer cuz there's no surface porosity to be sealed.