Planning to use lauan

If using epoxy for your scarfs, don't bother with any precise methods, in fact it's better if it's a sloppy interface. Rough it in with a belt sander or large disk sander (buffer or angle grinder), check the fit. I use 16 to 40 grit for these operations and I don't clean them up, except to insure they nest nicely when stacked.

Make a simple jig to align them as you glue them up. A couple of boards, nailed their width an a couple of brads is usually all that's necessary.

Don't over clamp, just enough pressure to insure contact with the goo, with ooze out all around, but not squeezing out all the epoxy.

If making precise fitting scarfs, for use with other glues or bright work, you can make a ramped jig for a router or plane. A hand plane after roughing it in with a power plane, plus some fine tuning with a sharp chisel can usually produce a nice scarf by hand.

 

Maybe don't work so hard??

Before you work too hard on this, see:

http://www.terryking.us/boatbuilding/Butt-Scarf-Joints-DaveCarnell.pdf

"Works For Dave"

 

I too have been using that joint for a number of decades, but I've taken it further into being fair. I use a large buffer/sander with a 10" disk (16 to 36 grit) and hollow out the are to receive the 'glass. The sanding pad, held at an angle, creates a shallow concave depression, which is filled with fabric and thickened epoxy. It will take some experience to judge the amount of goo and cloth you need to fill the depression, but once done and placed under plastic sheeting material, the cured result needs little to no additional fairing. The best part is the panel thickness is the same, so no hump to knock down.