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I'm taking an online woodworking course (The Weekend Woodworker), and built a small patio table a few months ago. I made it out of select pine (kiln dried), spray painted it (~2 coats), and put in on my back deck. Being the first thing I've ever made, I was (and still am) pretty proud of my efforts! However, I noticed today that the top of the table is already cracking:

Cracking on table top

What could be causing this? The deck is south facing, and gets a ton of sunshine all day, so perhaps it's a UV thing? I also note that water sits on top of the table after it rains, as if the top boards are cupped a little (I did indeed sand the top). I try to dump the water off each time this happens, but I'd like to know what (if anything) I did wrong.

  • Short answer — your pine gets damp, then dries out and as always with wood the ends dry faster than the wood further in and this leads to the potential for cracks to form. These are called end checks if you want to Google for more. The board ends that we can see in the photos, they're not the from-the-factory ends by any chance? Re. the cupping, this is natural and you can't prevent it. – Graphus Apr 28 at 4:25
  • BTW for exterior applications, horizontal surfaces in particular, two coats is not generally considered enough. You might want to prime, undercoat and then use as many as 3-5 coats for maximum protection, but, it depends a lot of the paint you're using. What paint is this? – Graphus Apr 28 at 4:26
  • I used Rustoleum 2X spray paint, and no, these aren't the factory edges. Not shown in the picture is the fact that these end slats are beveled at 45 degrees, so the wood is thinnest where the cracks begin. After doing some reading online, it looks like I should have primed before I painted. Is there anything I can do to prevent them from getting worse? What did you mean by undercoat? – Jonah Bishop Apr 28 at 12:33
  • "so the wood is thinnest where the cracks begin" ah, that alone may account for this because in addition to having end grain exposed adjacent to the surface the wood being thin along the top edge dries much faster through its thickness than the full meat of the boards. There are a few things you could try to halt this, but first job would be to address the existing cracks. Undercoat is not widely used any more, but it's basically an intermediate paint that goes between primer and paint that is intended to ensure adhesion (although many primers these days are designed to provide this). – Graphus Apr 28 at 14:47
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The end grain is much quicker at aborbing water than either the face or edge grain. So the end of the board sucks up the water. The board tries to expand, but the dry wood further from the end constrains it's motion. So the force crushes the wood fibers.

Board dries out, and the wood shrinks. But now the crushed fibers take up less room. Wood splits.

As the wood splits, water can use the splits to get in further. So the splits gradually move in from the ends.

Prevention: Saturate the ends of the boards with whatever finish you are using. Sometimes cut ends are in difficult to reach places after construction. I suggest doing as much of the finishing before assembly as possible, as this will also take care of places where boards overlap.

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