My first thought was that it was a fire retardant or something similar. The truck was a one owner farm truck. I scraped about a 1/4 of it off and the cab is in great shape behind it, no rust or holes.. It’s just a pain to get off. I wasn’t sure if this was something from the factory or from the original owner. I'll have to take a pic later tonight. It’s definitely a black tar like substance but hard.