This stuff is not apshalt ype undercoating, it is paintabel rocker guard coating based with naptha,toluene and mineral spirits, much like many rust paints.
It has a mild odor when warm, I am going to ther cover over it, or paint on a coat of Tremclad rust enamel. Infact I have done a test spot, it paints over well.
The product label, and instructions do not warn about innner cavity usage. It is fully cure in 24 to 48 hours, then paintable right over.
I will try a few things first and see where it's at, if nothing works, will as per Tim Adams scrape it off.
Painted over, then sealed with a insualtion sound deadener, then a headliner, may work..........Fred