Hi D,
From my short experience of dabbling in the noble art of box making, I would say it all depends on the size of the box, obviously a small box say 150mm x 150mm would look out of proportion with 20mm thick sides, conversely a box say, 300mm x 180mm would be disproportionate with sides 5mm thick.
As a good starting point I use 12/13mm for a box 300mm long and work up or down depending on size, not by a lot but the box has to be in proportion, if you have any thin 12mm strips then stand them on edge and see what sort of length looks proportionally good by using a couple of bits of ply gradually spreading them away from each other along the length of the strip, you will know when it looks right and usually if it looks right then it is.
As for the box size proportions, they go best if you follow the general rule of;
A. Golden rectangle...Start with the depth then the length is 1.62 times the depth and the height is 0.62 the depth (depth being front to back)
B. Double cube....The height and depth are one third of the length
C. 1:2:3 Box....the height is one third the length and depth is two thirds the length
This is just a guide and I find laying out some strips till it looks right serves just as good then when you have a pleasing shape transfer it to paper and here's where you can judge the thickness of your sides.
Sanding
Make sure you sand the insides before glue-up, I stick masking tape around the edges of my inside glue lines to prevent glue getting on the interior,swine to get off when dry and it will show when finish is applied.
I use Abranet, expensive to start but outlasts every other abrasive and therefore cheaper in the long run. I start with 150 grit then 320 and finish with 500, sometimes going on to 1000 depending on timber, Walnut for example, takes time but the quality of finish is worth it.
Hope my limited experience is of some use.
Regards.
Andy
