Solution to miter gap errors.
-Take the time to perfect the setup of your table saw. Blade parallel to the miter slot as close as you can get it and don't quit until it is almost zero. Sometimes the blade isn't really quite flat, in chich case, use a blade stiffener.
-Switch to a full kerf blade, thin kerf deflects during the cut because you are pressing it at a 45 degree angle. My favorite is the Freud Glueline Rip blade.
-Make sure the blade is EXACTLY 90 to the table and that your zero clearance insert is the same height as the table all the way around. Sometimes sawdust builds up under an edge of the insert and raises it.
-When setting up a jig or miter gauge, make sure it is 90 to the blade. Not 89.97.
-Lock the blade height and recheck the tilt (far right side on most saws). If it moves when you lock it, reset it until it is exactly 90. Get a Wixey digital angle gauge.
-Forget using a cheap miter gauge. Incra makes some very accurate ones. Fit the miter bar so it does not move at all side to side, but still slides easily in the slot. -Then recheck the gauge is still parallel to the blade. Some saws shift slightly when you lock down the adjustment
-Move the fence out of the way.
-Put self adhesive sandpaper on the face of a very flat piece of hardwood as your sacrifical fence. About 80-100 grit.
-Get some accurate corner clamps or a band style clamp.
-Trial fit first.
-Get an engneer's square, a good one, or a really nice draftsman's square (Rockler makes a nice one).
-Glue up and assemble, band clamp and double check the corners are SQUARE. If not you probably didn't check the dry fit well enough. You can make slight adjustments.
-Get some Timber Mate wood filler to fill in the inevitable tiny gaps.
I added anothe step. My wife bought me a Lyon (type) miter trimmer, which is like a horizontal gillotine that cuts a very precise 45 degree edge. Trim off about 1/16th to 1/32nd inch only. You can also cut perfect 90 degree joints with it, Cut almost to length, but leave double what you want to trim off. My trimmer came from Grizzly through Amazon. The blades will slice your finger off if you try to lift the trimmer through where the blade slices. It has a handle on top. Use that. Mine is mounted on a sheet of plywood and I bought the long arms with stop block for bigger frames..
Put it together as described and get out the Timber Mate, because nothing and no one is perfect. The Timber Mate comes in many finishes. Rub it into the gap with your fingers, sand and finish the frame. I most often use a semigloss or gloss wipe on poly.
If you want to make sanding easier, get a set of sanding blocks (Rockler) and use 3M's flexible sanding medium. The 3M is far better than paper for this purpose, so I prefer it. I go up to 220 grit, tried 320 but it didn't do any better. If you're a glutton for punishment or a perfectionist, use a curved scraper.
Here's a bunch of pictures, you can tell which is which.
1 commercial frame clamp
2 shop made frame clamp
3 Miter gauge, higest precision
4 Wixey digital angle gauge
5 Shaped sanding blocks for curves and coves (set)
6 3M flexible sanding medium Wonderful stuff, get it!
7 Lyon type miter trimmer worth it if you make many frames,
Every frame shop has either this or a more comples model
8 Sharp angle flat shaped sanding block (set)
9 Timber Mate, best filler I've ever used. Available in many wood varieties, Amazon
