Aircraft wood spars are often scarfed and the resulting joint is just as strong (if not stronger than the wood itself). I typically go about 15:1 by running a router mounted to a wide base plate over two sloped rails straddling the board at the end being scarfed. Works pretty slick.
I have also scarfed 1/8 aircraft plywood... that was a real beotch using a belt sander to create the feathered edge. Thankfully I only needed to scarf about 16" on the four parts needing to be joined.
This main wing spar here is made of laminations of perfect tight grained quarter sawn douglas fir boards. It has several scarf joints buried in the layers. It is good for over 10 g's positive/negative. I glued it up using West Sytem epoxy.
(I have a 35mm pic somewhere of my scarfing operation, but I don't know where)
