Here is how the forest service describes the wood this arbor is made from, "The wood of eastern hemlock is moderately light in weight, moderately hard, coarse grained, uneven in texture, and inclined to splinter when machined. Although moderately low in bending strength and shock resistance, eastern hemlock can be used in construction for light framing, sheathing, subflooring, and roofing. It is much used in the manufacture of boxes, crates, pallets, and as paper pulp for newsprint and other low quality papers."
This soft wood is not hardy and does not weather well.
Knowing this, I began by sanding the pieces of the arbor before putting on a better protective coating.  I found a number of spots that had been filled with white filler putty (see photo).