I assume you are trying to join the cedar on its long dimension.
You can miter the joints. If you are joining the long dimensions,
it would be best to have a tablesaw, though it can be done with a
The other choice would be to set a solid piece at the edge line.
You never said what thickness cedar you were installing, but the
solid piece would need to be the same thickness or slightly
thicker. You can run a round over, ogee, chamfer, or other trim
bit if desired. Let your tongue and groove meet this board. It
may require removing the tongue for a clean fit.
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