I've been using abrasive cloth from Klingspor (www.woodworkingshop.com) for my Great Auk 14 project.  They have a "bargain box" containing ten pounds of industrial grade abrasive cloth for about $20.  The pieces are large enough for long-board sanding and can be had in a mixture of coarse, medium and fine.  The abrasive is very good and lasts a long time and there's enough in the box to finish several kayaks.  I rip it to size and glue it on a 1/8-in by 3 x 32-in plywood board with contact cement.  Check it out, it may save you some money and the frustration of cutting sheets of sandpaper for your sanding. 

 Jerry