On Monday, February 13, 2017 at 5:31:39 PM UTC-8, Doug wrote:
Need to fill a split in an exterior board - what brand(s) of wood filler perform well in
this application after priming and painting?

If it's under zero stress, maybe you can get away with putty (I like Dap glazing compound).
Better, though, to replace the board. You don't have any control of the interior
moisture, it's gonna move after you apply any patch.