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In going around my house I notice that not infrequently the miters between moldings around doors and other places have opened up into a crack.

If I am putting new moldings in, how do I avoid this from happening?

I can join the moldings, for example, by using a spline, but that only goes so far, eventually a point is reached where you can't spline.

One idea I had is that it is a matter of dryness. If the wood has any moisture, then it will shrink when the moisture dries out eventually. Therefore, if the wood is thoroughly dried before being oil painted, then it should theoretically be stable because it is dry and and there is no way for moisture to get past oil paint, so the wood should never change size. Is that the only way?

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    Not an answer because it's too mundane, but: Two words: quality paintable caulk.
    – jdv
    Jan 28 '20 at 14:41
  • @jdv The caulk dries and cracks faster than the wood does. All the joints I spoke of in my question were all caulked by the original builder.... and they all opened up.
    – Treow Wyrhta
    Jan 28 '20 at 15:49
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    Oh, that sounds like terrible caulk. Quality caulk for trim will remain quite pliable and allow for quite a bit of movement. Of course, it can only bridge so much movement.
    – jdv
    Jan 28 '20 at 15:58
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While the molding pieces do not get shorter pulling the joint apart, shrinkage across the width of the molding can create a gap. I recently removed some 'modern 2 1/4" moldings from a door and discovered that in addition to nailing the trim along the length of the face to the rough framing, the original installer also used finish nails on the sides near the joint on the outside edge offset about 1" from the corner point. These nails held the joint tight for at least 50 years. (Be careful not to get too close to the corner or you will split the trim.)

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