Name that stain

Better yet, how do I “get it gone”? 2%?-nope. 6%? negative. Oxalic? Nein. EBC? Net.
Whiskey,Tango,Foxtrot. I’m wondering if it might be from tar paper.

It’s rotted…needs painted.


So, rotted up behind the window trim? I can’t say I’ve seen this before.

If its tarpaper, Id try the double eagle.