I am pulling up my sheet vinyl to replace the flooring with Ceramic tile. I have been using a scraper to pull up the old glue however, there is still some grey hard paste that is stuck to the concrete foundation. Do I need to get all of the old glue up or can we smooth out the new adhesive over it? If I do need to get the old stuff up, how (besides back-breaking hand scraper)? Thank you for your help.