Hello, I have a fairly old home. When I purchased the house, half the basement was covered with 9x9 tiles. Before I moved in, I removed all the tiles with a scraper, most just popped right off. Now I have this black mastic or adhesive left on the floor. I am looking to finish a section of the basement for a kids play room/exercise room. My contractor said to leave the mastic on the floor and go over it with a sealer, like the Kilz you use on interior basement walls for water proofing.

Any other suggestions on encapsulating the mastic?

Thank you