The Mirka DEOS 353CV (81 x 133mm) and Mirka DEOS 383CV (70 x 198mm) backing pads are not interchangeable.
An extension pad is available for the Mirka DEOS 353CV, which measures 81 x 165mm and enables the user to sand further into narrow places.
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Which backing pad fits my tool
Using an incorrect backing pad can cause additional strain on your sander. We have a useful guide to help you choose the correct backing pad.