What CAN'T or SHOULDN'T I use "Amazing Goop" Plumbing Contact Adhesive and Sealant stuff on? I want to use it to seal an unglazed toilet tank hole with a grommet of unknown material (some type of elastomer that has turned putty-like where exposed long-term to San Diego, CA tap water)

YOU CAN use it on porcelain fixtures such as toilet tanks.

BUT this grommet you are talking about... any chance we are talking about a toilet tank bolt and rubber washer? There are either two of these or three of these bolting a toilet tank to the toilet bowl.

If you are talking about these bolts you are better off replacing the bolts... not real hard to do if you have some basic hand tools... and we are glad to talk you through installation.

Let me know what exactly you are trying to do... if you are just trying to cover over the hole with bolt and washer it will fail one day... probably while you are at work or on vacation.. and can make a real mess!

Do you own or rent the property where problem is? If you own.. do it right, fix it. If you rent, call the landlord... Anyway, get back to me with any answers you can. Thank you... Mark