and yeah, when i find something to clean that brown gunk i will
Toluene works, although they'll suspect you of having a P lab when you want to buy a couple of litres of it

Can get it from industrial painters usually. Just take in a suitable container.
Someone mentioned a citrus based cleaner in one of the old "Dry ice/underseal removal" threads which I don't know if it was on this current version or one of the lost forums.