Regarding butt joints with a backer board; I haven't done it on a kayak but have on a furniture mock-up using 1/4 inch plywood.  The edges of the backer board 'telegraphed' through to the outside of the curve; I could see where they were from the outside of the piece.  That was just a mock-up so not important but, personally, I wouldn't do it on a kayak.  Scarfs aren't really that tough.  Sharpen up your plane and practice on a few pieces of luan; you'll be fine.

If your boat has very shallow curves, it'll work, but  a butt block joint won't bend as fairly as a scarf. With thin wood and/or a sharp curve, it'll give you the effect that Jack mentions above.

Laslo

We seem to have a language barrier.  Are you British?

Kim

Many kayaks are made with butt joints.  CLC's puzzle joints are butt joints.  A strait butt joint is fine as long as it is supported by an extra layer of fiberglass tape and epoxy on one or both sides.  Backer boards are not usually used.  This will show through as a slight bump if not carfully feathered, but does not seem to interupt the radius of a curved panel as long as it is not too extreme.  This adds an extra step or 2, but having done it both ways I have found butt joints to be simpler and easyer to line up.  I would post a pic but can not compete with Laszlo.

Laszlo you scamp, I want one of those weights.

Custom butt joints would be cool.  But I started with scarves and I will finish that way. I will play around with making butts. 

After trying all the above methods I like using my plane the best.  Preferably sharp. 

Thanks to all