You admittedly could have made the miters tighter and lessened your finishing problems. With veneers you can't round of the corners more than to just take the "knife" edge off. Otherwise you could work out a way to add a solid piece of wood into the corner. It might even be a contrasting wood for decoration.
What will your final finish be? Will you paint it? Stain it? and top coat with varnish, poly, shellac, etc.
Most fillers do not take a stain the same as the wood but water based fillers come closer than solvent based which can be nonporous.
A common mantra in wood working for things like fillers and finishes is to do some testing first.
What will your final finish be? Will you paint it? Stain it? and top coat with varnish, poly, shellac, etc.
Most fillers do not take a stain the same as the wood but water based fillers come closer than solvent based which can be nonporous.
A common mantra in wood working for things like fillers and finishes is to do some testing first.