Hey everyone, i have a driveway cleaning and sealing coming up soon. There is one spot by the front porch at first glance i thought was just scum, dirt, and just built up crude. Upon closer inspection it is hard scale i would call it. Not sure if it is old driveway sealer that has been built up and broke down too much over the years or what? I tried a test spot to see if it would come off with just heat and pressure with no effect at all. Any help on how to approach this would be great?!?!