I've recently refurbished and replaced some sections of my timber fascias and box ends etc.
On the new sections i treated all sides with 2-3 coats of this.
Existing timbers where the paint was flaky got a thorough rub down to expose the wood and then applied the treatment.
Make sure you give cut ends a good soak for a few mins.
Its fairly quick drying which helps at this time of year and soaks right in.
I suppose you shouldnt strictly need to treat the reverse side as It shouldn't really get exposed to the weather but i did it anyway for good measure.
I used the same treatment about 2 years ago when refurbing/replaceing some badly rotted sections of a timber conservatory frame followed by three coats of sadolin superdec.
So far its holding up well, still looking fresh.