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Step 4:  Screwing up and Fixing the Scarf Joints
(not an official step according to the instruction booklet)

Well, I went to check out my scarf joints after a day of drying and the good news is that the bottom panel scarf joints came out nicely.  Leaving me with fine looking sturdy 16 foot long pieces:

The bad news was that the side panels didn't turn out quite so well.  Now I want anyone who reads this to benefit from my mistakes so here is my analysis of what went wrong:
I keenly deduced that this was not what was supposed to happen.  The main problem with sanding it down is that the plywood is three-ply and I sanded through much of the top layer leaving the middle ply, which had its grain running parallel to the joint, as the primary means of strength.  Not a good thing.

To avoid this I would advise not trying to sand the scarf joints so they are smooth to the touch.  Just sand off any excess epoxy and say "good enough."  Also I would try to avoid placing them too close together to begin with but that was not the fault of my system for lining things up but rather the way in which I lined things up.  I am not sure how it could have been avoided because I spent a long long time trying to line them up perfectly.  The moral:  Strive for perfection but not with 120 grit on 4 mm plywood.  

So to fix it I decided to turn it into a "butt joint" which is where you simply abut the two pieces up against each other with glue in between them and then place a piece of wood behind the joint (on the inside of the kayak) to strengthen it.  I should actually end up with a stronger joint after all this.  Here is the size of the piece of wood I am talking about:

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