Armstrong Tools
C.J.,
Keep your hands off the power tools for the scarf joint, lest you scarf it up.
I just finished my eigth joint (second airplane), each takes less than ten minutes with a file. They are flat, even, smooth. The trick is using a big, flat mill file with a safe edge, then a smaller edge cutting file for dressing the fillet. For milling, the wider the file the better, and in the proper reflected light you can easily see the plane left by each stroke so you can make adjustments as you go. Mark the cut line with a piece of masking tape; it sits proud and won't get hidden by shavings as a marked line will.
(Files cut one way - pushing away from you. Don't saw, your arm's joint articulation will make a curved line. My tool-n-die maker father could file a part to shape so well it looked like it was machined.)
Set the edge even with a flat table top and stroke from edge to center of the material. Other way around your knuckles raise the file at too obtuse an angle. The exercise is not to hide the joint, but remove .025 from the leading edge of the .032 inboard sheet to lower the .025 outboard sheet's leading edge even with the .032 aft edge if the tank. (10 knots worth less drag!) Prettiness comes from even, straight lines. Gap width, constant gap width, isn't as important. (True of any joint - wait until you get to the cowl!) The length of the scarf area I make about an inch; more doesn't enhance anything and makes maintaining a straight line more difficult. Use the small file to cut the fillet. Dont' try to make a square notch - that's a stress riser. Instead slightly radius the under side edge of the top sheet to snuggle into the joint. Scotch-brite and spot prime.
I caution staying away from hand held power tools because they are nowhere near as controllable, in an instant can remove too much material from the wrong place (this is thin stuff you're working with and your target is only .007 thick), and this fine work requires finess, not horsepower.
Captain John (another one!)
Siebold World Airline (That's right, singular.)