Cutting perfect miter joints is in most cases much tougher than you think. You have to measure perfectly and cut all the parts perfectly. If you get only one side slightly wrong, you will have a gap. If you are installing molding in your home, always first measure the corners because they are rarely perfect 90°. You are probably wondering how do people make perfect miters if it is so hard? Well, in most cases miters just look perfect because the gaps are well concealed.
How to fix gaps in miter joints

In this article we will teach you how to fix gaps in miter joints and make your miters look perfect with a few little handy tricks.

The first method is the most obvious one and the best one if you want a permanent result. The best material for filling gaps in miter joints is real wood. It is best to use this option when the void is large enough and regular in shape. If the void is irregular you can straighten it out with a back saw or dovetail saw. Use a chisel to take a sliver from another piece of matching wood, but be sure to make it deeper than the gap. Rub the sides of the sliver on sandpaper until it fits the width of the gap perfectly. Then force the glue into the opening with a knife or a piece of paper and slip the filler into place. Wait after the glue dries and then use the flat face of a chisel to trim the protruding filler piece flush with the surface of your project. Follow that with light sanding and you will have a perfect miter joint.

The second method is filling the gaps with wood fillers. You can buy various brands of paste-like wood filler that will disguise smaller gaps. The downside is that it won’t take stain like wood, but a repair that runs parallel to the grain will blend in nicely.

The third method is somehow strange, but it really works in some cases. You can really close the joint in a matter of minutes using nothing more than a round-shafted screwdriver. Just rub the screwdriver shaft back and forth across the joint, applying just enough pressure to compress the wood fibers. The general idea is to slightly round over the corner and conceal the gap. Normally that will work only on small gaps but it is really neat inexpensive trick.

How to fix gaps in miter joints with screwdriver

The method with finishing nails is also very effective for small gaps in miter joints. You just need to clamp the mitered corner together with enough pressure so that the corner comes together and any gap that was present is closed. This method works best on trim pieces that have opened up once installed, or on mitered frame corners that have a gap in them. After the gap is closed, hammer a finishing nail through one side of the mitered corner into the other side, so that it goes straight through the mitered joint. Then hammer a second finishing nail through the other side of the mitered corner into the side you nailed through first. In that way the cross nailing will hold the mitered corner closed once the clamp is released.

The method with back saw or dovetail saw has also proven very effective for small gaps. Just firmly clamp the members of the joint at the angle needed and gently run a fine tooth saw (a back saw or a dovetail saw works perfectly) down the joint between the two pieces. When finished, slide the members of the joint together and fasten in place with glue or nails. If your gap is larger then bring the two members of the joint together and run the saw between them again. Don’t worry, in most cases, you will not create a fit problem for the other joints by removing a tiny bit more material at this corner.

How to fix gaps in miter joints with hand saw

The following method is very common when making a new joint, but it can be very effective for gaps on existing miters also. First measure both edges of an unattached mitered corner to find the end that is closest to a perfect 45-degree angle. Then hand-plane the edge of the second-best mitered end, shaving off very small portions at a time until the two mitered ends match perfectly. Hand sand the mitered edges and once you have sanded the miter to a point where it mates well to form a corner, you can connect the lumber into a mitered corner.