Sand

Most people use a random orbital sander (ROS) to smooth their work. Fairing Boards and a Speed Files are two other effective sanding tools. Both are simply long sanding boards. It's surprising how much wood you can move with one of these tools. Some builders make their own fairing boards. Speed files (and long sandpaper) are available at Auto Body Supply stores.

Sand is also the stuff that gets into your kayak when you launch from a beach. Getting it out seems to be a little harder than getting it in, especially in a SOF where it tends to reside between the stringers and the skin.