How should I apply polyurethane over paint to ensure a durable and smooth finish?

To apply polyurethane over paint for a durable and smooth finish, follow these steps:

1. Ensure the paint is completely dry before applying polyurethane.
2. Lightly sand the painted surface to create a smooth base for the polyurethane.
3. Clean the surface to remove any dust or debris.
4. Apply a thin, even coat of polyurethane using a high-quality brush or roller.
5. Allow the first coat to dry completely before applying additional coats.
6. Sand lightly between coats for a smooth finish.
7. Apply multiple thin coats rather than one thick coat for better durability.
8. Allow the final coat to cure fully before using the surface.