Only a two part epoxy type substance will have a prayer of holding, and even that is not a permanant solution. Porcelain over cast iron is basically glass that's sprayed on in powder form and then heated to liquify. When it cools, it's a solid glass coating over the steel. Glass, although strong, is brittle, as you've discovered. And, glass is very very difficult to bond anything to. Once the porcelain chips and the iron has come into contact with air and water, it's gonna rust and eventually that repair is gonna come out. The good news is that it takes a really long time for a cast iron tub to rust enough to become a leak issue. The bad news is that spot will be ugly (and expand) the whole time until you can't stand it anymore and replace it.

We bought a used Nordic hot tub with a Gecko controller and an Aquaflo Flo-master XP2 pump about 10 years ago and it recently stopped working the controller was blank the led on the spapac board was on the fuses were good line power was what it should be: so I bought a topside controller. that was not the problem the spapack was bad. Replaced the spa pack and I had a lot of noise and bubbles coming out of the jets with the air ventures off and the controller errors out with FLO. I adjusted the pressure switch until I got a FLC error and then backed it up 1/4 turn. I then noticed a piece of a disintegrated impeller ware ring in the old heating element. I took the pump out and cracked the impeller shaft. I got a new wet end from poolpartsdotcom and still had bubbles in the jets. I replaced all the seals in the new wet end and still got an air leak. my symptoms are pump turns on no bubbles for a period of time(10 to 60 seconds) then bubbles, then FLO error, after 4 or 5 cycles of the pump I get a HL error and the top surface of the heater is hot.. If I take out the filter there are less bubbles and maybe it will take 8-10 cycles before it trips to HL. I don't see a water leak (area is clean) and I cannot hear the air leak (I tried listening with a stethoscope and a pipe). My better half is loosing patience. This post was edited by sean.jackOfSome on Wed, Feb 19, 14 at 22:27

I can't tell what that thing is. I'd take it out and have a look at it. Might be some sort of check valve. The heavy vinyl tubing at the home centers of the same inside diameter will work just fine.

Your builder or contractor will have to contact the tub manufacturer to get information on their recommended repair techniques or to schedule one of their certified repair vendors. When done, it should look like the brand new tub it is. Feel free to tell your builder/contractor that you are agreeing that they can try the repair, but if you aren't satisfied, they're on the hook to install a new tub including all the tile surround repair, etc. When we had our master bath remodeled in 2014, one of the plumbing geniuses went into the crawl space underneath our bathroom and drilled a hole up through the floor right through my new tub. Then, of course, he lied about how it happened. In any event, the contractor got in touch with the manufacturer and had a certified repair company work on the repair. I can't see where the original damage was at all. It looks and performs just like brand new. I hope this helps.