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In terms of filling a surface as abraded as a staircase I might try the repaircare fine surface filler. It's a 2 part but it's designed to comply with a moving surface and not delaminate when it's thinly applied.
Otherwise Toupret ready mixed wood filler or some kind of gorilla glue and wood dust.
I'm just about to fill nailheads in some floorboards and I'm thinking toupret wood filler.
Yay nearly finished!
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