This is one of several stress cracks opened in this spa skimmer making daily refills necessary.
For a temporary repair, clean the surface of the skimmer with Acetone or a grease cutting dish soap. Allow to dry.
Frame each crack by placing duct tape parallel to and approx. 1"away from and 1" longer than each crack.
Apply 3M-5200 Marine Sealant (available most home supply, hardware store, any marine supply) with a paint scraper or other flat tool, about 1/4" thick.
When the sealant begins to skin over in about 20 minutes, carefully pull and dispose of the tape.
Clean-up is with Acetone. Allow 24 hours before you refill the pool.
Ultimately, this skimmer will have to be replaced and that's detailed on the Pool School PRO CD, as well.
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