I have just finished bandsawing my 1/2" baltic birch bows for the hatch lid. Sanding the outside of the curves was no problem, since the flat base of my portable belt sander does that job just fine. However, when I built my first tear, I hand sanded for hours to rid the inside of the bows of the bandsaw marks. This time, I tried a new trick that I have never used in my 55 years of woodworking.
I cut a curved pine filler with matching 'hooks' on both ends, to hold it in place against the shiny metal base plate. The sanding belt stretches easily over the curved member, and after placing some pencil marks across the surface to be sanded (to let me know where more pressure should be applied), I slicked out the inner curves with no problem. In the event that any of you have never tried this little trick, give it a go, I should have tried it years ago.
Roly, five days into the new build, and all that is lacking is the hatch lid and the two doors. Should have that done in the next two days. :-)