When I shape the bottom with a long sandingblock, it naturally tends to shape concave instead of a flat bottom contour. How is this possible?! I use a long sandingblock 23’’ - 4’’ and a bit less than 1 inch thick with 60 grit
wrapped around it. Any advice?
Mook, If you are holding the block at around 45 degrees to the stringer, due to the rocker your block sands the centre more than the edges. You might want to try a few different sanding patterns, 90 degrees to stringer, etc. Others will have better suggestions, I’m sure.
Every shaper has different sized sanding block, some for the same purpose.
I never liked the long sanding blocks, as in my day, the board all had belly on the bottoms.
Could your block be flexing?
I’d use only a 12"er for finish work, otherwise the bottom was sanded with a 8"er.
Keep track of the # of strokes.