I work a lot with 3 mm ply and it is difficult to get a really good scarf joint as the ply tends to curl slightly at the edge, and small errors are more important in thin material. I now use a belt sander, with a steel bar under the edge of the ply and another behind the cut and parallel to it. The belt sander stops cutting when the belt contacts the bars and leaves a perfectly flat cut with a feather edge. The pressure of the tool keeps the ply flat. I am going to try rollers.