Once fully dry, it will provide a waterproof barrier. But it must be applied to a surface that is, and will remain, dry. Any damp, dust, oil or grease on the surface to be painted will prevent proper adhesion, and lead to blistering or cracking later.
Some paints are advertised as "allowing the wood to breathe". This is a con - they've taken a disadvantage of the paint and pretend it's a useful feature. With wood, you want to seal it completely. Any moisture allows movement in the wood through swelling and contracting, so it really needs to be completely sealed. Enamel paint will do this.