PCB conformal coating / potting / resin in an IP65 case
If the enclosure has any penetration or mating surface, it will ‘breathe’ due to the change of temperature and pressure. IP65 is not hermetically sealed.
Depending on your application, you could use a breather, desiccant or conformal coating. Also consider corrosion of metal items in the enclosure. Also consider the effects of the conformal coating to your RF sections in terms of dielectric and the possibility of shrinkage putting stress upon components - especially with RF.