Remove the existing hardware from the cabinets. Take the doors off the hinges to make applying the polyurethane finish easier.
Apply a very thin coat of polyurethane onto the cabinets. Use a paintbrush and paint the finish on with long, even strokes in the direction of the grain. Let the cabinets dry overnight.
Sand the surface using a gentle scuffing motion with 320-grit sandpaper. Wipe the dust away with a tack cloth.
Add another coat of polyurethane and let the cabinets dry overnight. Sand again and apply the last coating of polyurethane to give the old knotty pine cabinets a new sheen.
Replace all the doors, except for two cabinets, after the varnish dries completely. Choose two cabinets that are next to each other, for example, above your stove, to leave without doors.
Fit the two cabinets with glass inserts to create an open appearance. Use etched glass if you have stainless steel appliances and clear glass to complement appliances with other kinds of finishes.
Add brushed satin nickel pulls and hardware to match stainless steel appliances. If you don’t have stainless steel appliances, use polished chrome. Use pulls rather than knobs for a more modern appearance.