I'm not sure how to describe it without doing some loading and measuring - but it's fairly stiff. The center is a piece of 1/8" aluminum plate surrounded by a piece of large "C" shape UHMW plastic that is probably 1/8 to 3/16" thick. The plastic is typically used as side rails for conveyor systems. It only takes a little pressure to bend to match the fore/aft contour bottom of the I20 hull but it would never match the side to side contour without permanent deformation. The whole thing is about 4" wide by about 12" long and I use adhesive backed sandpaper (on a roll) which I found at Lowe's in three different grits (120, 220, and 320). The tall handles allow me to give the board incentive to put more pressure in one area over the other - but I only did that when I was initially taking down a high spot in some filler and wanted to use up a spot of sandpaper on the board that wasn't yet clogged. For finishing I would simply grasp the outer 2/3 end / sides of the board just inside the handles and let the center and ends find the contour of the hull.
The board used for the r/c plug was a piece of 1/4" corrogated plastic backed with a 1/8" piece of PVC plastic sheet and it had no handles. I had to play with some different combinations until it "felt" right.
The boards need to be stiff enough that they don't dip into any concave points as you sand on a hull. They need to be able to 'bridge' those concavities and be able to translate the curvature behind and in front of the board as it moves. On the other hand, it needs to be flexible enough to easily bend to match the contour of the hull fore and aft (the axis with the least curvature) and when you use it, you always keep it oriented in that axis. As you sand on the hull with the long board, you'll quickly be able to see where the low spots are and you can mark them and fill them. Then shave down the filler to get it close to the hull contour and start with the board until you have a very smooth and uniterupted contour. I've always been pretty intimidated by this kind of work but with a little practice, I now feel pretty confident about achieving something pretty refined.
