Reply by David Craig

I have flattened planes before that were somewhat concave. I lowered the grit, when flattening, to make the process a little less time consuming. Take a marker and run it up and down the plane. When you start flattening, you will probably see the marks on the front, back, and sides go away while the hollow remains. When the marker is all gone, it is pretty much flat. Move to finer grits from there.

David

-- There is little that is simple when it comes to making a simple box.