A loin is what loin chops or center cut pork chops are cut from.
There is very little fat to start with. The only time i have smoked loins i have stuffed them .
Unless there is some really hard fat , i wouldn't trim before smoking .
I'd smoke it fat up . Reason being that way what fat that does render will flow over the meat b4 falling into the pan that catches the jucies( You do have a pan ?)
Brining ? in my opinion ,No . Again ,in my opinion Injecting adds more flavor.
Here is the way i would smoke a loin, paint on a mixture of 1 part mustard , 1part molasses w/ 1 tsp liquid crab boil.
coat heavily w/ my rub . Wrap w/ Tightly in plastic wrap . Place the loin in a pan , Make up an injection of apple juice ,butter , a shot of spiced rum and some brown sugar.
heat the injection untill the butter and sugar or melted and let cool.
Inject the loin thru the plastic wrap and then store in the fridge over night.
Unwrap and dust w/ more rub and let it sit while you prep your smoker.
Smoke at around 230 till internal of 160 . Foil and rest for at least one hour b4 slicing.