I decided to go the route of using reclaimed XPS for my interior insulation. (2" XPS). After washing, scrubbing and cleaning these panels as best I could they are still quite dirty - but I don't think I can reasonably get them any cleaner. I don't particularly mind the grime because they will all be behind a wall anyway...
However - how "sticky" should the sheathing tape be when applied? I am concerned that the dirt and dust left may leave me with an inadequate seal after applying the sheathing tape. I just tested a piece it once applied it seems to lift off easier than I was imagining.
This is a "clean panel" - I have cleaned it up a bit better than seen, but it does give you an idea of what the panels look like.
And this is the tape I bought.