Which of the following methods is recommended for ensuring a smooth finish?

Applying multiple thin coats is recommended for achieving a smooth finish because this method allows for better control over the application and reduces the likelihood of runs and drips. When each layer is thin, it dries faster and more uniformly, resulting in a smoother surface once the final coat is applied. This technique also helps to build up the color and achieve the desired depth while minimizing imperfections.

In contrast, applying only one thick coat can lead to uneven drying, sagging, or an undesirable texture, as the thicker paint may not level properly before it begins to dry. Skipping primer can result in poor adhesion of the paint, which can affect the finish's smoothness and durability. Using untested materials poses a risk, as they may not perform well or interact negatively with other products, leading to an unsatisfactory finish. Thus, utilizing multiple thin coats is the ideal approach for a polished outcome in painting and refinishing projects.