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I hope this is clear. So part of an outdoor project I'm working on has a thing like this:

enter image description here

  • Those are all 2x4's.
  • The vertical piece on the left is joined with pocket holes, not shown, not relevant.
  • The blue/gray areas are 3/8" diameter counterbore holes, about 1.75" depth.
  • The red things are epoxy coated deck screws.
  • All faces including the ends are finished.
  • The very bottom piece is pressure-treated fir, finished with exterior grade primer + latex paint.
  • The other two pieces (the vertical and the piece with the holes) are untreated fir, finished with oil-based stain + exterior grade spar urethane.
  • The holes face up; water could theoretically sit in them.
  • All the pieces shown in the diagram are flush and you're looking at the wide faces of the 2x4's; there's no secret depth component to that drawing aside from the holes being drilled on the centerline of the wood.

My problem is this: I did drill the counterbore holes before finishing, but I didn't really get any urethane down into the holes. I put the whole thing together, then it rained, then it occurred to me that rain can get into the counterbore holes, where it comes into contact with raw wood. I observed that the water beaded up just fine on the finished surfaces, but soaked into the wood in the holes.

Now, my questions are:

  1. Do I really need to seal up these holes? As in, do I really need to? Of course we don't want water soaking into the wood but given the relative size of the hole, and the fact that it doesn't rain constantly (i.e. there's time to dry, except in the winter if snow piles on), will I truly regret not finishing the holes?

  2. For hole B, it's pretty easy to do regardless. I could fill it with putty, sanding the finish off the top a bit for the putty to bind, then refinish that area. So regardless of question #1 I'll probably still do hole B.

  3. For hole A, this is actually the one I'm more concerned about. There doesn't appear to be a gap between the vertical piece and the horizontal but I wouldn't trust it to be water tight. The problem is I really don't want to disassemble that corner to fill in the hole, so: Could it still be a problem? Is there another way to seal that up?

The way I figure is, even if water gets in there, it's such a small surface area that it will eventually even out through the surrounding wood and ultimately dry back out through the hole.

For deep counterbore holes on exterior projects exposed to rain, is the effect of not finishing the inside of the hole negligible in the long run? If not, what will happen? What can I expect to see in a few years?

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    If the raw wood is not protected, it can rot. If it can rot, it will rot. – Ast Pace Sep 27 '16 at 20:28
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    Terminology: those are counterbores, not countersinks. Yes you really need to protect the wood in the holes because water getting into the wood there will undermine the finish on either side, leading to premature failure. Any gap or hole in an exterior finish is a problem, a deep one is a major issue because it gives access to end grain, which is like a sponge as you know. – Graphus supports Monica Sep 28 '16 at 8:42
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You might wait until its pretty dry, and use some exterior caulk. You can fill hold B, and seal around the bottom of the stud on top of hole A.

Not sure what this is for, but yes, over time moisture will be able to wick into the end grain through the holes.

No it probably won't be drastic, or even noticeable for awhile.

  • Well, I don't know what the long term effects would've been, but I took your suggestion for caulk on A (good idea). I filled B with putty and refinished it. Easy enough, better safe than sorry I guess. – Jason C Sep 28 '16 at 5:29
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On top of other concerns given in comments and answers like inevitable rot I would be concerned about your winter climate.

I would be more worried about something like ice forming in those area and putting pressure where it was not meant to be. While doubtful of its significance there could be the potential of causing damage from expansion.

Depending on the project you don't really need to do much to fill it. Plug cutters should get you a tight fit. I made a sandbox for my kids and used indoor glue (I know!) and cedar dust to fill the holes. Still looks good after 4 years.

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