To seal a bathtub overflow drain to prevent water leaks or prepare for a waterproofing test, you must remove the overflow faceplate and replace or seal the behind-the-wall gasket. Unscrew the two screws holding the overflow faceplate, pull the linkage assembly out, and inspect the black rubber gasket behind the tub wall. If it is deteriorated, replace it with a new tapered rubber gasket. For a temporary exterior seal, apply a bead of 100-percent silicone caulk around the perimeter of the faceplate. Callout type="warning" title="Hygiene Warning": Ensure the overflow pipe remains unobstructed so the tub cannot accidentally overflow during normal baths. Verification: Pour water around the sealed faceplate and inspect behind the wall to confirm zero leaks.
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