Using a flat white paint product called Killz on the inside will help lots to protect from water damage and prevent mold. It is made for use in wet areas. A solid coat of varnish over that will make it pretty much bullet-proof as far as water is concerned.
I have (had) the JBJ Duo pushing maxijets with the Algae-Free Maxi-Jet Upgrade kit mounted on the Algae-Free magnetic powerhead holder. I ran the first set for two weeks, and the two mounts snapped within 24 hours. Got replacements from Mark at Algae Free, and they both snapped within two...
