Corners of wood picture frames sometime need reinforcing after being damaged by falling off the wall, being generally mishandled or having their glued joints dry out. Any of these things can cause the mitre joints in the corners of picture frames to loosen. The mitre joints are made by cutting the framing pieces at 45-degree angles, then matching them together to make a square corner. A tight mitre joint makes an attractive corner, but a loose joint is unseemly and detracts from the picture on display. There is a simple, effective way to put corner joints right.
Place the frame facedown on a work surface. Detach the picture-hanging wire from one of the eye screws. Open up the back of the frame. Remove the picture and glass and set them aside.
Balance the empty frame on one corner. Grab one strip of the frame carefully but firmly in each hand, at opposite corners.
Wiggle the two frame strips back and forth to loosen up the mitre joint. The joint will have two to four nails holding the corners, depending on the size of the frame, which will cause some resistance. Pull the joint open only far enough to squeeze some carpenter's wood glue into it.
Lay the frame face up on the work surface. Push the two strips of framing back together until some of the glue starts oozing out of the joint. Wipe off excess glue with a damp rag.
Place a corner wood clamp under the mitred corner. Tighten the screw on the horizontal section of the frame moulding to firmly lock it in place.
Tap the adjoining vertical strip of framing close with a hammer or rubber mallet to snugly close the mitre joint. Tighten the remaining screw of the clamp.
Tap the corner nails with a hammer and nail set to drive them deeper into the wood and make the mitre joint even tighter.
Repair one weakened joint on the frame at a time if you have a single corner clamp. Give each reglued joint at least 24 hours to dry before removing the clamp.
Lay the repaired frame facedown on the work surface. Place a flat metal corner bracket in each corner of the frame. Secure the brackets with the provided screws and a screwdriver to finish reinforcing the picture frame's corners.
Cover up exposed nail holes with wood putty that matches your frame.