If you have a small gap on the outside of a miter joint, here is a cool trick to help close the gap without using any wood putty.

Take a screwdriver or any other piece of rounded metal, and push it down along the length of the miter joint. This will cause the wood to round over into the gap. Keep rounding until you have the gap filled.

If you want some insurance that the gap will not open back up, you can put some wood glue into the gap before you round over the joint.