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
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.