Why is it that when I try to join this polysurface to a planar surface, I get this strange meshing result. More importantly, how do I get a clean join here?
There seem to be no naked edges.
This is typically the mesher choking on the edges.
One way to solve it is set a custom rendermesh for this object.
For windows you would do this ( not sure how that would be done GUI wise on a Mac)
"The most important setting here is max distance edge to surface. "
For this specific case i recommend the detailed settings, and use 0.01 mm for max distance edge to surface. Then the strange extra triangles disappear.
you may play arround with the other settings as well.
In my experience a maximum distance edge to surface of 0.01 for the render mesh is much smaller than needed with most geometry, and results in meshes with much larger number of faces than required. I typically use a maximum distance of 1, and very occasionally 0.2. I also usually set a maximum edge length, typically around 4 to 6, and a maximum angle of 1.0 degree.
Expecting some non planar edges, I deleted the circular surface and tried to cap the rest but it failed. Also some sub-surfaces on the parallel ribs are faulty. You can see through them partially. Not good. Rebuilding from scratch is what I would do.