How I Cut a Scarf Joint in Plywood
I use a power planer to cut scarf joints in thin plywood, and it works pretty well. Here is how I do it.
Like most approaches to scarfing plywood, mine cuts both pieces at once. I lay the two pieces of plywood together on top of a piece of scrap wood at least as wide as the joint plus ½—¾". Then, I use wire brads to nail a fence of scrap stock ¼" or less thick along the rear edge of the cut. The fence should be long enough to project beyond the plywood being scarfed, and should be supported by slips of scrap stock to keep it level to its end. Finally, I adjust the rabbeting guide of the power planer so the sole of the planer sits lightly on the plywood with the guide pressed tight atop the fence stock and the side of the sole pressed against the edge of the fence stock.
Cutting the joint
With the depth set to about ⅙ the thickness of the plywood, I set the planer on the stock and fence, making sure the front of the sole is against the plywood. I make sure the planer remains level front to back and against the sole for the entire stroke. I repeat the stroke until the outer edge is very thin, and then adjust the depth so the last couple strokes remove only a little material.
Advantages and disadvantages
The big advantage of this method is that it produces a very smooth, clean joint without requiring a lot of skill (because I don't have any). On the down side, there are holes left behind by the brads, and the beginning of the cut tends to be a little inaccurate for the width of the cutting blade, until the rear sole is on the plywood.