I've struggled with this problem for years. It is a segmented plywood wood or laminated wood garage door that's raised vertically by a torsion spring. The paint at the bottom of segments (particularly the next to the bottom one) cracks. The cracks are roughly vertical. I've repaired this 2-3 times in the past, but it keeps coming back.
I'm fairly sure the problem is that rainwater seeps into the seam between two segments and runs down the plies. I've tried to seal the upper edge of the segments with paint but it never stays. When the door is all the way up or down the segment edges come together (touch) and the paint sticks to the other edge and is torn/pulled off when the segments separate while opening/closing. I've tried sealing the edges with this stuff and injecting it into the paint cracks before repainting
Here are the best pictures I've been able to make to illustrate this problem, but they don't make it clear.
If I can't come up with something this time that works I guess I'm going to have to replace this door that's in good condition otherwise.
Anybody have any new ideas?