My guess is its plain old roofing tar.
Won't wash off...
Sand blast was only way I got the metal clean.
But it took forever to get thick layers off.
I waz able to get the worst off with a good dose of COLD weather to make the tar brittle and then using an assortment of scrapers and elbow grease chipped away at it till the majority was gone.
Then blasted what was left.
That was the "fastest" way I found.
