On The Run OTR.970 Hard To Buff Ink - 210ml refill bottle is an alcohol based ultra permanent black ink. Filled with classic Hard To Buff ink, this is one of the best and strongest alcohol based inks available. It's an incredibly hard to remove deep black ink that is glossy and highly opaque. This ink will mark most surfaces and, as the name suggests is highly permanent. Suitable for refilling all On The Run Hard To Buff Markers. Also compatible with a variety of markers, pens, mops or even apply directly from the bottle with other applicators.