The picture below on the left shows a close-up of the surface of a carbon fiber and epoxy resin laminate. On the right is the same surface after sanding. Sanding exposes the fibers in the laminate, creating lots of places where glue can seep between the fibers creating mechanical adhesion.

In the original picture of the sanded laminate you can almost distinguish the individual carbon fibers, which are about 5 μm in diameter.