I have an attached drywalled garage w/ a subpanel that feeds several outlets in the garage. All wiring is in surface mount PVC conduit using THHN wiring, and all are serviced via GFCI breakers. I want to install an outdoor outlet / in-use box on a wall about 10' from where one of the outlets is. What is the code requirement for getting from a surface mount box inside the garage to a box mounted outside? Can I just run conduit & THHN wiring from the existing surface box to a conduit body that punches into the wall, and run THHN wiring from out of the conduit body and into the outdoor box? This would mean there would be a small length where the THHN wiring was "exposed" ( in the wall between the conduit body and the box). Is this OK, or is there a better way?
THHN must be entirely contained within conduit...
Your approach won't work because NEC 310.10 requires that you use insulated conductors as part of a Chapter 3 wiring method, instead of standing on their own.
but, you can use a stub length of conduit from the body to the box
However, outdoor-rated boxes generally have hubs built into them that accept conduits of an appropriate size. As a result, you can fit a stub length of PVC conduit (often called a "nipple") between the conduit body and the box, provided you're using a plastic weatherproof box. (These are often called "FS" or "FD" boxes.)