Priming siding and trim on site is a losing battle, learned the hard way

Tried priming a whole job's worth of siding and trim by hand on site and it was miserable. You always miss the back faces and the cut ends, which is exactly where water gets in later. Last job I just had my supplier prime everything at the mill before it shipped, backs and ends included, and it saved a ton of labor. You still seal the field cuts on site but that's a small fraction of it. If you're doing any real volume of trim, the factory priming pays for itself.