A mounting adapter including a bearing for rotatably mounting a surface processing tool to a tool holder body. The bearing has a rotatable hub including an attachment in an upper portion thereof for non-rotatable attachment to the tool holder body. The mounting adapter includes a rubber flex pad having a periphery and a first aperture. The mounting adapter includes an element supporting the periphery of the rubber flex pad, the element having a second aperture in vertical registry with and beneath the first aperture. The mounting adapter includes an end plug within the second aperture for closing the first aperture. The end plug supports the flex pad from beneath the second aperture.