I think the big question has to be how you intend to finish the
floor and what the floor is (concrete or wood subfloor).
If it is concrete:
Razor blade scrape the floor and cap the previous adhesive.
Is the left over stuff black? - cutback adhesive, possible
asbestos. Cap with ARdex floor leveler or similar.
If it is wood subfloor:
If it will be ceramic tile, I would cap it with wonder board or
If you are applying any other type of flooring, I would suggest
capping the existing wood floor with sheet goods - mdf, plywood,
hardie board, etc - DO NOT BE STINGY WITH GLUE AND NAILS.
You can also skim out the adhesive area with one of the Ardex
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