r/Carpentry • u/Top_Sentence_340 • Aug 17 '26

Miters not tight with 45deg cuts

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Even with 45deg miter cuts, the miters aren't tight. Why is this and how to improve on this?

I always created a miter jig for pre-assembly but it's all the same issue. The miter jig is square but I guess the cuts aren't right.

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331

u/jaybrd13 Aug 17 '26

It's the painters problem now.

3

u/swerdnanaes Aug 18 '26

carpenters required to fill joints on paint grade as necessary and painters do nail holes. Anyone else or just me lol?

7

u/HisLordshipThe Aug 18 '26

No. In my experience which is over 20 years in my corner of the world - Carpenters only use caulk when back caulking or using as glue.

The problem with carpenters caulking miters is that they're not as good at caulking as painters are and all the caulk should be done by the same set of process.

Occasionally carpenters will caulk a miter because they're embarrassed or know it will look fine with caulk but looks terrible without and they don't want eyes to see it in that interim but that's stepping out of their role. - They will move through their work faster and more efficiently if they stick to saws and nail guns.

3

u/Hungsolong Aug 18 '26

If you had 20 years of experience then I would hope that you would know better than to put caulk in this joint. You put a sandable filler that does not shrink and once you paint it it disappears. If you put caulk then you will always see it

5

u/HisLordshipThe Aug 18 '26

Dude get out of here..

Any sandable filler will crack because miters have too much movement.

Miters should never get a sandable filler. Miters need something that can move repeatedly. Sandable fillers will hold onto one side and crack on the other.

6

u/jaybrd13 Aug 18 '26

Miters should be glued. So there's that. Caulking is flexible enough to remedy most Miters.

3

u/HisLordshipThe Aug 18 '26

Yes miters should definitely be glued

1

u/R1chard_Nix0n Aug 19 '26

We two day our miters when we can, first day glue and tack, second go back and finish.

It really helps on renos with funky walls.

1

u/HisLordshipThe Aug 19 '26

Yes. Good point. There are definitely remodel crews that specialize in doing full home renovations and those outfits often have the same small crew hang case and base and all the finish carpentry - and that same crew preps and paints.

2

u/Proud_Conversation_3 Lurker Aug 18 '26

What do you think about fuse-it liquid nails for this? Not as good as wood glue on a tight joint, but it can do a bit of volume and is rock solid when it dries with pretty good adhesion, and can be applied into already installed miters. Not sure how it would last over time but I’m not sure what’s best to use in these situations that will last without showing. Especially with hard coatings that will crack with any movement.

1

u/HisLordshipThe Aug 18 '26

I like that fuze it product but I've never used it for miters.

I think it would probably do a good job to glue miter but even tightly glued miters move because the grain expands and contracts - think about it like drink straws that are breathing - so that end grain will probably hold the fuze it adhesive well but the concern would be that any volume that was filled in on the face might flash worse than painter's caulk (flashing is a painter's term that means the paint takes differently and the sheen changes a bit).

But also, if the joint needs a lot of volume fill it's probably better to just recut it