How can I fix a gap in a miter joint when building a picture frame?

[Evelyn9]

I tried to make a simple wooden picture frame using mitered corners, but no matter how carefully I measure and cut, there's always a tiny gap at the joint. I've been using a basic miter box and a hand saw, and I'm wondering if my technique is off or if I need a different tool to get that perfect fit.

[EthanHM]

I feel you. Those gaps usually mean the joints aren’t sitting flush when the glue goes in. For me the issue was the cut not being truly square once the stock was clamped and the kerf sneaking the angle off. I stuck with the miter box for a while, then started sanding the back faces a touch so the corners could seat, and I glued with clamps across the long edges too. It helped a bit, but not perfectly.

[AdamHG]

Could be a wood movement thing more than the angle. If the frame bends after you add glass, the gap can show up or disappear. I’ve had frames look tight dry and then humidity stretch them. Check the stock for warping and support the back while you clamp.

[PaisleyZL]

I did the same setup and the inner faces weren’t perfectly flat, so the corners wouldn’t meet flush. I shaved a hair with a block plane along the edges, re-glued, and used tight clamps. It finally lined up, but I wouldn’t call it rock solid.

[StellaT]

Maybe the real problem isn’t the joint at all. The backing or the glass being off can fool you into thinking the frame is off. I once finished a frame and the picture wouldn’t sit square until I squared the backer first. Not sure if that helps your case, but it shifts the pressure in the right direction.