Which action is recommended to improve conditions before paving over existing asphalt?

To enhance the conditions before paving over existing asphalt, applying a primer to the existing asphalt is a recommended action. The application of a primer helps to ensure better adhesion between the old surface and the new asphalt layer. This is essential because it provides a bonding layer that fills any air voids and can help to reduce the chance of future failures such as delamination or stripping.

By creating a chemically conducive environment, a primer aids in the integration of the new asphalt with the underlying layer, enhancing overall performance and longevity of the pavement. This approach not only improves the bonding but also acts as a barrier against moisture infiltration, which could undermine the integrity of the pavement.

In contrast, the other options may not offer the same benefits. Removing all existing surfaces might be more disruptive and costly without necessarily guaranteeing better outcomes, while using a thinner overlay could lead to inadequate structural support and possibly shorter lifespan. Roughening the surface can improve bonding but might not address all adhesion needs as effectively as primer application.