I don't see what harm 'glassing the scarf backs would do save for asding a bit of extra weight, and not much of that if you're careful in the doing.

OTOH, if you've done scarfing before, have confidence in the strength of your work w/o the extra layer of material 'glassing'd add, you can probably go with the unbacked joints with little to worry about.

Are there scarfs in panels that'll see more twisting or bowing during assembly? Maybe you can selectively evaluate which parts would stand to benefit, leaving the others alone? Surfaces in tension would benefit more from the added strength than those in compression during assembly & later use.

For what it is worth, I built a Shearwater Double from kit in 2015.  That manual (also from Eric Schade) does not call for plans builders to back the scarfs with glass but does call for kit builders to back the puzzle joints.

If I were you, I would not back the scarfs as long as that area will eventually be glassed over because it will save time, weight and expence.  The savings is certainly small, but it is there.

Honestly, I do not even glue puzzle joints with thickened epoxy anymore as long as both sides will be glassed.  I just CA the joints together and they get plenty of epoxy when the glass is wet out.  All of the strength is in the glass layers anyway.

ditto mark's comments....with an emphasis on "...as long as both sides will be glassed".

it's pretty clear the entire outside gets glassed....so it's more about the insides and making sure that is covered.  if the area is not going to be glassed on the inside, i would put the glass tape over that inside joint.

the shearwater 17 is a very pretty boat.  i built one from a kit a number of years back.  it also paddles like a dream.   you are really going to enjoy it

h

   Thanks to all  for the reassurance - I have stitched the hull together and will be glassing both sides anyway so will use that layer to add strength to the scarphs.

Great looking kayak H - I've a long way to go but that is great inspiration!