filling crevice on exterior trim boards

I'm going to install some new trim on the exterior of the house, and I've cut various wood pieces to the right sizes, but there will be thin crevices between some boards even when nailed, ranging from 0 to about 1/8 inch. WHat kind of caulk or putty should I sandwich in there while nailing the boards into place? I'd like to put in some excess stuff and then wipe it smooth/flat with a putty knife. Thx in advance.
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