Over the last two weeks I worked on scarf joints between the overlapping portions of the inboard and outboard skins on both wings. The idea behind these scarf joints is to taper the thickness of the skins where they overlap. The top surface of the bottom skin gets tapered so that the edge is about half the thickness of the rest of the skin, and the bottom surface of the top skin gets a similar taper. When the two surfaces come together on the wing, the overall thickness of the joint is reduced. The whole joint isn’t scarfed, just the portion within 3″ or so of the spar.

Here’s one scarf area marked out on a skin, with some duct tape in place to protect the places I didn’t want to taper.

Preparing to scarf a wing skin

Some buiders use sandpaper or files to taper the skins. As you can tell from the picture above, I used scotchbrite discs in my die grinder. By varying the grade of disc and grinder speed, I was able to control the amount of material removed. I worked outward from the duct tape, increasing the grinder speed, pressure and dwell time as I approached the corner.

Here’s one finished taper. After the sanding was done, I cleaned and alodined the area using a Touch-n-Prep pen. Worked great.

Preparing to scarf a wing skin

Here’s one finished scarf joint – if you didn’t already ‘get the flick’ on how this works, you will now. This is the joint where the inboard and outboard skins overlap on the left wing’s upper side. Remember, just the forward 3″ or so of the joint is ‘scarfed’.

Preparing to scarf a wing skin

This was a time-intensive process…tapering eight skins for upper and lower scarf joints on both wings. But in the end, I think the results were worth the extra effort.