How to fix gaps in miter joints

How to fix gaps in miter joints

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. 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 - 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 - 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.

Thank you for sharing, Handyman tips team!
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30 comments

  1. Anonymous

    If you lay your trim ontop of the other trim board, then mark where they intersect they always come out perfect.

  2. MWycombe

    When you have an old Victorian home (we do) very few of the angles are exactly 90 degrees square. So, the 45 degrees for the common miter doesn’t work that well – especially if you are doing crown molding. The approach I have found most satisfactory is first measure the angle – a digital protractor ($15) works best. Then, using high school geometry, compute the size of the bisected angle and note carefully which is the left end and right end angles. Cut your first length and cut a small ‘key length’ from scrap wood for the next turn that should mate with the first piece. Work your way around the room, cutting a short ‘key length’ to check the correct mating of the miters at each turn around the room. For modern houses that are usually very square this is not necessary, but for old wooden Victorian houses, which are rarely square, using a ‘key’ cut from scrap wood ensures the best possible mating at each turn of the room.

  3. Phillip

    Try to cut your longest runs first, so if you goof it up you can see what you did, use another piece and do it right, and save that goofed piece for a space where you can cut it shorter.

  4. Curtis Delong

    Listen if you got through high school, and you passed your math , and you check all your angles and use a chop saw and set the angle on the saw to the reading on the square, you can get it right even if you chcpeck yourself with a short piece of cut off first. If it doesn’t fit tight, don’t continue with the thought that it will be ok because it’s not ok.

  5. Aad

    Make a paste of glue and much sawdust where you’ve worked with. Spread into the gap and shed it as the glue has dried. It wil work in most of all the cases. However, when you are a cabinet maker ( like
    discribed in one of the comments) it’s quite an insult if you making daps like this!

  6. Tony

    If you can’t cut a perfect mitre by hand then your only option is to buy an electric chop saw with a 45% angle cut, as long as you are confident enough not to cut your hand/fingers off, you still have a problem if the corner you are mitring is not square so it’s best to check that first before attempting to cut the mitre.
    There is no such thing as a Professional Handyman? there is no recognised qualifications in the name of Handyman you are either a Carpenter or Joiner or Carpenter & Joiner or Cabinet Maker, all handyman are non qualified and to call one of the professionals a handyman is taken as an insult

  7. Jd

    Yeah this is all for tree person tag doesn’t do it everyday, I have zero problem cutting a perfect mitter with 7″ + crown or base.
    But I will tell you that a lot of what your saying is way harder than just caulking it and painting. A handy man won’t do strain grade because he knows he can’t. So just throw some caulk in it and paint it.

  8. Bob Knapp

    If you have no other choice, out of matching stock, a method I Will use is to remove piece and sand the molding’s back tapering to feather out away from corner. Usually will need to do same on other piece. Most times you gain enough to even re cut the miter. Methods mentioned all have merit for painted work. Been finish carpenter 40 years. As others menyion, it is a good idea to cut inside and outside template pieces to check first b r fore cutting


  9. Author
    Handyman Tips

    Every method has it’s advantages and everybody prefers a different one! All of this methods work and we are confident that your method is also effective!

  10. Bob Carpenter

    No no no. The best way is to lay a piece of thin wood under the piece being miter, so when you cut your angle the expose side, the front, looking at you will be on a bevel and your 45. Here. If your wood was rubber then roll your miter down just a hair and do the other side to. The fronts will meet perfect but the back of the miters will not but you can’t see that and if you can by looking down, fill, it will never go anywhere. Plus, make up some short pieces of miters to test to see how far they are out. This is the best way for old and remodeling. New should be better but maybe not, so lay a shim down on the miter box and lay your piece of trim on top and cut your miter and this will give you your bevel 45 degree cut.

  11. Jim

    This is really a homeowner tip, not for a craftsman. Some of the methods may work with surfaces that may be painted but hardly for staining. More concern on quality is paramount, no need for filling/patching new joints.

  12. douglas a hanlon

    fj doyle…really you walk on water? so let’s say you in a medium quality house and down to your last stick of wood and since you are almost perfect every other miter in the house looks like it grew there…but it’s late and your supplier is closed and the job is forty miles from home and the supplier is ten in the other direction and you cut a mitre that has a 32nd gap…are you telling me a finish carpenter of 35 years that i am amateurish for using that trick with some glue and light sanding?….must be nice to be independently wealthy and a perfectionist with no concern for making a living(btw that’s why your wife hates you)

  13. Fjdoyle

    The screwdriver method should never be used. It’s very amateurish and rarely does more than crush the end fibers of the joint.

  14. rex

    Agreed Marc proper tools and knowledge of how to use them, there should be no reason to have a large gap on a miter joint.

  15. Dave

    Used to mix PVA glue with the same sawdust as the timber and fill the gap with that. Sand it smooth when dry.

  16. Fantastic Handyman

    One does not need to be the ultimate handyman to make good miter joints. I won’t say it’s easy and you don’t have to be a pro handy man, but still if you’re motivated, crafty enough, and you can afford the time investment, it will surely be a sip of helpful experience. When I have to take a job like that, I usually go for 6d nails, unless the miter joint / flooring doesn’t allow it. Of course, if we we’re talking about professional handymen, you should be capable of handling all 3 options, since different clients wan’t joints done differently. As a carpenter I would always avoid glue but in some cases that is the one and best option to get the job done.

    Dmitri,
    fantastic handymen

  17. Marc Friedman

    The other alternative is, get it right the first time. Unfortunately, their are enough people in the trades who do not understand the practical application of high school geometry. With over 40 years of building under my belt, I’ve seen it all the time, for years. If you know how to bisect an angle, a clean miter shouldn’t be an issue. That assumes that the equipment you are cutting with is set up properly.
    So the argument for using filler, other than nail holes, is nonsense. I would recommend gluing the miter. When it comes to putty,
    The epoxy wood putties that come in different colors are a good choice for gap filling. They can be tinted if you know how.

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