PVC conduit into PVC box: Can I make it watertight when using a threaded fitting? Are sealing locknuts allowed?
Glued fittings are generally always used for PVC conduit to PVC boxes.

Most people also misunderstand the concept of exterior conduits and fuss about "keeping them watertight" when, in the code, they are defined as a wet location. And they will be, every time, no matter how watertight you make them - by condensation, if by no other means.