Join Date: Nov 2005
Location: Indianapolis IN
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If the choice still exists, I also would vote for butt joints instead of miter joints, just because the clamping and holding is a lot more straightforward.
In either case, I would recommend "biscuit joining" as the way to make the joint. If you want to strengthen after the fact with an L bracket, then go ahead, but what you need is an overlapping wood joint bonded with carpenter glue or something stronger. Since your pieces sound like they are already cut to length, you can just as well make the joint on the miter cut, it's just a little more interesting to hold in place while the glue dries.
If you do a web search on plate joiner or biscuit joiner, you can find what you need to know about tools, materials, and methods, including videos on how its done. The basic description is like the old mortise and tenon joint (round peg in round hole) but it's been simplified in that you only have to be precise in one axis instead of two. It has the added benefit of aligning your two pieces in one direction so you only have to clamp in two.
A biscuit is a pressed wood oval. They come in different sizes. Biscuits are about 1/8 inch thick and 1-2 inches in overall length. You need a special tool, about the size of a hand drill, to make the oval slot for the oval biscuit. Probably you have a wood-working friend who can loan you theirs. I'll loan you mine if its easier, just PM me. The way it works is you adjust the saw blade so it hits the middle of the thickness of your frame material. Plunge the saw on both pieces (backing up your frame piece with another block of wood -- don't ever use your hand!), and then drip glue in both slots, shove the biscuit in one side, assemble, clamp, and wait. It's really strong. I can make some pictures tonight and post them if that's helpful, just let me know.
Good luck with whatever method you try next, I'm sure its aggravating to have the first round move on you.