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An ideal caulk joint forms a hour glass shape with the thicker sides being attached to the materials and the thin more stretchy middle not being attached to either side. This give the sealant a
An example of this would be using a thin and thick rubber band. Cutting the length of each to about one half inch long, you can easily grip the larger band, but its difficult to stretch because of it being thicker in middle. Take the same length but thinner rubber band you can pull much easier and it flexes quite well.
Now take this same condition with a sealant, but using the thicker or larger surface being attached to the siding, and the thinner section being in the middle. Now you have the best of both worlds working for you.
In the case of the siding Butt Joint, your working with a thin surface to begin with, and seeing the edges of the surfaces are pretty close if not touching themselves its impossible to get a good surface attachment without creating a thicker center where the sealant will be pushed behind the open joint. Thus a early failure of the sealant joint.
I would try to push a thin aluminum strip up between each joint area. Put a 180 degree bend on one end and make the length of the strip long enough so when you push it up it reaches the top of the board under it. Thus allowing the metal edge that is bent over to catch on the edge of the lower siding section., This in turn will keep the metal strip from falling out. This will prevent any water from dripping behind the siding and eliminating the need for any sealant. The strips only need to be about one inch wide or so and should not be too difficult to cut, bend and slide up under each butt joint on the siding.
Hope this helps.
Your only concern here is to see how much of an overlap was done when it was applied. if you can see the tar paper, plywood, or what ever was put on the wall before the siding went up near the
If you still see the under course of the siding and its going up beyond the bottom of the course above meaning you cannot see the top edge of the board below I would simply clean out the old caulk and not worry about it as the bottom board is protecting the wall.
The bottom line is there is no sealant unless you coat over the top of each board, which would be ugly that will flex enough and still hold onto each edge of the board without adhesion failure.