Single boards are almost never wide enough for even relatively narrow applications such as door panels, nightstand sides or headboards. Thick glue lines are usually areas of weakness because most glues lack internal strength. Thick glue joints also form noticeable straight lines that detract visually from the appearance of furniture.
The board edges left by machine jointers are pretty good, and are used without improvement in factory production. Slight changes in pressure and speed as the board is feed through the jointer causes dips in the edge, as will bits of sawdust thrown onto the jointer table by the machine. These dips result in areas of thicker glue lines.
Woodworking craftsmen of earlier times always dressed the machine-produced edge with a long hand plane to further refine it. Even with today’s improved machine jointers, my experience is that several passes with a jointer hand plane are always needed before a full length shaving is obtained.
After hand plane jointing, I always check that the edges are still perpendicular to the board faces. Sometimes I’ve found that hand planing introduced a slight change from a 90 degree angle, too slight to see with a T square but noticeable when the boards are dry stacked as in the above picture.
A call to Lie Nielsen, the hand plane manufacturer, suggested I pay attention to the position and pressure applied by my thumb at the toe of the jointer plane.
In this picture, my thumb is over the left corner of the edge. The shaving will be slightly thicker under my thumb..
Pencil lines were drawn across the edge before a shaving was taken, moving the plane from right to left.. In about the middle of the picture, I moved my thumb from the right side of the plane’s toe to the left side. While faint, pencil lines remain along the left corner of right half of the board, and along the right corner of the left half of the board, showing how the shaving was effected by nothing more than moving my thumb.
While this may sound like much ado about nothing, when done properly refining the edge with a jointer plane takes only a minute. Eliminating a glue line may contribute to a piece of furniture becoming a family treasure that is kept for decades. And working to one’s best is a never ending quest to better understand and practice essential skills.