How can I effectively refinish old hardwood floors with gaps to restore their beauty and durability?

To effectively refinish old hardwood floors with gaps, start by sanding the floors to remove old finish and smooth out imperfections. Fill in the gaps with wood filler and sand again. Apply a coat of stain to match the desired color, followed by multiple coats of polyurethane for durability. Allow each coat to dry completely before applying the next. Sand lightly between coats for a smooth finish.