You will need to turn off the water to the toilet and drain it. Let it dry and then use an epoxy sealer.
This product worked for the crack in my toilet tank: https://amzn.to/2vWZZX8
Hope this helps
Turn off the water supply to the toilet.
Drain the tank of all water.
Dry the tank until it's bone dry (both on the inside and outside).
Apply plumber's putty or silicone sealant on the cracks.
Wait the time recommended on the sealant manufacturer's instructions.
Turn on the water supply to the toilet.
Here's a step by step to guide you:
Step 1 - Disconnect the Water Supply to the Tank and Drain It
Step 2 - Dry Out the Inside of the Tank Completely
Step 3 - Apply the Silicone Plumbing Epoxy
Step 4 - Let the Silicone Epoxy Dry Thoroughly
Step 5 - Refill the Tank
Hi Jag: You could try that new product Flex Tape, or even use Aqua Seal Tape. Home Depot carries both, and you can use them under water. Good luck
Two part epoxy putty would be the best on the inside of the tank. For the outside you can use porcelain appliance paint.
Flex Seal can be used on almost every surface: wood, metal, tile, concrete, masonry, fabric, glass, plastic, aluminum, porcelain, drywall, rubber, cement, and vinyl. Plus, it won't sag or drip in the summer heat, and it won't crack or peel in the winter cold. It even prevents corrosion.
The tank will eventually break, causing water damage to the bathroom. ... This is because the bowl bears all of the weight of not just the toilet, but anyone using it. This could cause serious injury and damage to the bathroom if it's not replaced.
I suggest epoxy putty. I use it to repair everything..but its intended for plumbing. Its an absolutelynamazing product.
It's a two-part epoxy sticky-found at big box hardware stores in the plumbing department near the pvc purple glue. In a clear plastic red capped tube. I get mine at Lowes $6. Home Depots its version and JB Weld is another but more $ and smaller. Other smaller 2 part epoxy sticks options can also be found in the boat marine and wood repair shopping sections.
You mix -press -then you can use scrape off excess (I use depleted gift card) and/or sand and paint. Cured putty is strong enough to drill through.
Putty safely seals/waterproofs for plumbing boat repairs.... Ive used it for exterior entry door frame wood rot repair, elliptical pedal repair, multiple arts and crafts repair of damaged pottery- wood -ceramics, plug up holes in planters, repair cracks in planters or make a new lip on the pottery planter. Ive repaired holes in a utility garage sink ( thanks previous HO) repaired a break in the top rim border of an elliptical peddle, repaired damages in a concrete statue, fixed a split wood disk on a wind chime, its a great defect filler.
check this pin https://www.pinterest.com/pin/536350636868478676/
This will help. https://sansone-ac.com/how-to-fix-cracked-toilet/
Flexseal or JBweld works well.
I covered one up for years until we replaced the commode. My sister in law knitted one for me using white yarn and matched the lid. She included a drawstring so it could be tightened...worked great. Hope yours is the tank lid as mine was.. Good luck