You could work your way through a few solvents to test - white spirit, acetone, (toluene, but that would probably leave a lot of trace).
There's brick cleaner, which is often just concentrated sulphuric acid. If that doesn't work, go the other way with sodium hydroxide.
With appropriate PPE, of course.
I’ve tried hypocholite and brick cleaner - with no joy. I may try white spirit and acetone. Thanks
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You could work your way through a few solvents to test - white spirit, acetone, (toluene, but that would probably leave a lot of trace).
There's brick cleaner, which is often just concentrated sulphuric acid. If that doesn't work, go the other way with sodium hydroxide.
With appropriate PPE, of course.