Cuprinol Ultra Tough Wood Filler is a tough and durable formula for repairing damage caused by wet rot and for major joinery repairs. Cuprinol Ultra Tough Wood Filler is used before varnishing, painting or staining bare wood to repair decayed wood. It may also be used indoors and outdoors on sound wood, e.g. furniture, panelling, etc. to fill knot holes, screw and nail holes etc. Cuprinol Ultra Tough Wood Filler hardens by chemical reaction. It is fast setting, non-shrinking, weather resistant with good adhesion to sound wood. It can be sanded, filled, drilled, screwed and nailed when dry.