How to fix a crack in the corner of an outside wooden lamp post

You'll have to do a little checking to determine if you can clamp that seam back together. It might have been a poor fit to begin with in which case you won't be able to clamp it back. Try getting a hold of a clamp similar to the one pictured below and see if you can close the gap. If you can, get some good exterior wood glue and have at it. If you can't close the gap, get some quality wood filler that's paintable and fill the gap. Then paint. A quick and easy fix would be to just caulk it but it looks like a nice light so do it right.

Picture from amazon.com

Jack has great answer and wow that clamp.

I don't have time to wait for that thing to come in the mail though - amazon has been taking 5 days for their 2 day shipping.

Get the drill out, at about 2 inches on either side drill a 45 degree pilot hole to the other side - needs to be straight obviously... take your time on this. After that get a wide bit and punch a hole right over that so your screw head fits inside.

You will need a 3.5"+ wood screw. Don't over tighten and don't go overboard on screw size. This should take 10 mins max.

Cover your small hole with some Durham's wood putty. With 2 mins of sanding and light touch up paint it can be 100%.

It is rather clear when looking at a closeup of the photo that the corner was not originally a nice tight miter joint. You can see that some type of wood filler material was used to fill the gap which was much larger out at the tip than it was down closer to the post.

I am on board with DMoore's idea of using a couple of large screws to try to pull that back together but it may be a good idea to clean out the old filler first and then after screwing it a bit back to the original gap and then refill with new filler. For outdoor use it may be applicable to use bondo to fill the gap but some sanding will be required depending upon how much care is taken to smooth out the bondo filler. Of course after the filler is applied let it cure and then repaint the whole thing when the whole post is nice and dry.

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