To prepare the roast, at least 24 hours before smoking, trim fat to 1/4". Rub the roast all over with coarse kosher salt. Place it in the refrigerator, uncovered, until ready to smoke (24-48 hours for best results).
Take the prime rib out of the refrigerator about 30 minutes before smoking.
Preheat smoker to 225° F.
In a food processor or chopper, mix together the butter, rosemary, thyme, garlic, and lemon juice until combined.
Spread herb butter evenly over the roast.
Place prime rib directly on grill or in a roasting tray on the grill and smoke until the internal temperature of the meat reaches 110°F (rare) or 120°F (medium rare) - roughly 40 minutes per pound. Remove from grill.
Heat smoker, grill, or oven to 425°F. Roast until internal temp reaches 120°F (rare) or 130°F (medium rare).
Remove from grill, loosely tent and rest for at least 15 minutes. Slice and serve.