[hawker445]

It's most likely some sort of paint stripping agent. It's mean stuff and strong, but probably not acidic in a chemical sense.

I used to strip antique furniture in a solution of sodium hydroxide, which is a base, and strips off paint in nothing flat, provided it was heated beforehand. There are other striping agents as well whose compound I cannot recall anymore, but they were very volatile and if you used them in a closed space you had to be careful not to inhale the fumes. Nasty stuff. Almost passed out a few times.

To do a job like that on an aircraft is one heck of a major undertaking. Hats off for the patience it requires.

You are absolutely correct