I'm bonding solid hardwood flooring over a concrete slab using Bostik Dura-Grip. A Bostik tech rep told me that I need to remove all of the black mastic where vinyl tiles were installed for their product to work properly. I've scraped up as much as I can but need go deeper to get the rest. I'm afraid to use a chemical remover because concrete is porous and insufficient cleaning afterward could prevent proper adherence.
What is the best way to remove black mastic (and tan mastic in one room) without chemicals?