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Is there a process for resealing the spaces between the floorboards (the floor is otherwise in good condition). Wood paste? Putty? Which would have a good shelf life that would not shrink on drying.

Wood is a living material that reacts to different factors: humidity and heat that make it "work". It swells as soon as the hygrometry increases and it retracts as soon as it drops. That's why, whatever the product used to fill the spaces, the repair does not usually hold: cracks always reappear. If the slits between the slats are particularly large, stuff them with papier-mache and some big hemp twine, then cover with wood pulp. Many refills are often necessary. All you have to do is sand the floor before you glue it off or varnish it

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