What condition indicates the use of a block sander versus a dual action sander?

The use of a block sander is most appropriate for fine feathering because it provides a controlled and even sanding action. Feathering involves blending the edges of the area being sanded with the surrounding surface, particularly when preparing a surface for painting. The flat and rigid surface of a block sander allows for better control, ensuring that the transition between the newly sanded area and the existing surface is smooth and seamless. This technique is particularly important in auto body work for achieving a professional finish.

In contrast, a dual action sander is more suitable for situations that require quicker material removal or dealing with uneven surfaces, as it combines circular and oscillating motion to cover more area quickly. While effective for initial sanding and larger surface areas, the dual action sander does not provide the same level of precision required for feathering. Therefore, for fine feathering specifically, the block sander is the preferred choice to achieve the desired results.