Upscale wrote:
....
Completely different applications though. The first is for box type
construction and the second is dedicated to joinery along the lines of panel
construction. Naturally, both benefit from substantial glue surfaces.

But I've never had a single case of even the factory finger-jointed
material not failing quite early in exterior applications. If they
can't make 'em work under controlled conditions of fabrication, chances
of any better luck for diy'er are even less imo.

I'd go w/ the scarf option meself; how one would make the cut w/ the
material already in place as I gather is the OP's intent w/ the router
is beyond my ken.

That said, if try it, the only thing that would have any chance at all
of holding more than a season or two at the outside would be epoxy or
resorcinol. I don't know what they use for the commercial stock but I'd
think it also would be something similar for exterior application but
whatever it is certainly isn't up to the task. My opinion is the joint
is unstable to shrink/swell and that causes the failure but that's just
that--I've not tried to make some for testing but just observed they
don't last. In fact, I just noted the other day there's repair work Dad
had done just before we moved back that have precisely that problem
that's needing replaced around the basement door frame...

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