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TiberiusSecundus posted on r/centuryhomes5w
It's glue from a prior flooring installation. Adhesive spreaders have 1/16gth inch teeth (or 1/8" or whatever) and leave that kind of mark. Could be very, very difficult to remove. Doesn't look like the black asbestos kind of mastic. I've used this to remove difficult mastic, and then sanded a lot after https://www.amazon.com/Wood-Deck-Removal-Tool-4-1/dp/B07CK57CCK/ref=sr_1_2?sr=8-2