What are the best techniques for applying putty filler to achieve a smooth and seamless finish on surfaces?

To achieve a smooth and seamless finish when applying putty filler, it is important to first prepare the surface by cleaning and sanding it. Apply the putty filler in thin layers, allowing each layer to dry completely before sanding it smooth. Use a flexible putty knife to spread the filler evenly and feather out the edges for a seamless blend with the surrounding surface. Sand the filled area gently with fine-grit sandpaper to achieve a smooth finish.