RE: Lanier Mariner 40 MKII ARF 62"
It is very important that the joiner be glued to the spars at the outer ends of the joiner. The center is not as important. The idea behind the method described by the instructions is that the epoxy you put in the closed pocket is forced up around the joiner as you push it in and fills the gaps. You should put most of the epoxy in the pocket and only enough on the joiner to wet the surface.
If you put the epoxy only on the joiner, most of it well be scraped off as you insert the joiner into the pocket, and you will not get a good joint at the outer ends where it is needed most.
Jim