The moment you start dealing with mitered corners especially when you are adding pieces of trim pieces to your project, you will notice that even a very meticulously joined mitered corner is likely to separate when you are installing it. There are several reasons as to why this happens, but the most likely one is that the pieces of the rim may be warped or twisted slightly.
How to Close a Mitered Corner which is open
This process can be a bit complicated, but if you do it a number of times, you will eventually get the trend. If you take a shelf made of plywood which is wrapped accurately by pieces of mitered 102 trim, you will notice that after the trim has been attached to the plywood, the bottom edge will be apart by close to 1/16 of an inch. The plywood shelf will eventually be stained despite it being a small gap which can be caulked in becoming a painting. If you caulk the gap, it would appear to look like a thumb.
The good thing is that there is a logical solution to this issue. By using a single pair of the 4d finish nails; you can start by lessening the gap with the help of clamps, drive a nail to pass through each trim board until it gets to the opposite side of the board. This is one of the common ways of holding mitered corners the moment the clamps are let out. To be safe make sure that you set the finish nails with the help of a nail set and fill the holes of the nails with wood fillers.
You might have a better chance at getting it right in disguising the filler if you keep on adding stain to the project at hand. You can actually start by filling the nail holes and then touching up the filler with some stain compared to the initial way of filling the holes then smoothen the filler flush.