Dear John.
Most people like for the building to show through otherwise they wonder what you are trying to hide with the trees.
However, for you and your desire, open the plant's GDL to see if there is any code in the 2D script for a fill. You need to look simply for the FILL command. If there is no FILL command then no matter how you change the part it will never obsecure the building.
Also check that the library part is foward of any other object. This is the move forward command found under the OPTIONS menu.
Perhaps a screen image of both the AC and PM windows of the elevation in the same picture would help us to help you better.
Terrence Sturm, Architect
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