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Why is it that DA wetsanding is not possible?


It's possible, just not advised.

It is extremely rare to find someone who uses a DA sander correctly. Most people (including "professionals") hold the sander at an angle that causes a gouging action and only uses a narrow ring around the outside edge of the sand paper. This causes ripples in the surface. Also, the backing plate in DA sanding pads is easily bent when the DA is dropped, or even stored incorrectly. This causes the sander to want to jump around which also results in ripples in the surface.

On a stage of the process as critical as the final wetsand, it's advised to do it by hand.