PCB Coating remover

[Archangel]

It looks like RTV silicone caulk. Good luck..... You should be able to peel most of it by hand. I would use an EXACTO Knife also...

Or heat sink compound. Is it rubbery or past like? If it turns out to be RTV be sure to not allow lithium batteries in that device, seems RTV & lithium combine to form a severe corrosive situation, FAA made folks replace ELTs many moons ago.

EDIT: On second look, I think Dave is correct. RTV. Now, the question is do you want to trim the edges or UNPOT the PCB? Stoddard solvent will turn RTV to jelly, do not know if it will harm anything else.