Water Heater Leak

We have an 11 gallon Isotemp 40 water heater onboard. The heater works off AC power when the boat is connected to shore power. When the boat is not on AC power, the water heater works off the engine hot water return. The engine hot water is at 130 degrees, so through a thermo-mixing valve, we get more than 11 gallons of hot water for doing dishes or taking showers.

A while back, we noticed one of the fittings to the water heater had sprung a small leak. The liquid coming out of the fitting was pink. The engine coolant is pink, so we found out the leak was at the fitting for engine hot water return. We didn't want to be without hot water for the weekend, and the leak wasn't too bad, so we put an absorbent pad under the fitting, to monitor. After this weekend, we decided to make this project a priority since a coolant leak could make the engine overheat. It was hard to tell where exactly the leak was coming from, but after we took the fitting off and cleaned the area, we identified the source of the leak. The coolant was leaking out of the threaded connection from the heater to the hose. We put a small plastic container under the fitting, and disconnected the hose from the heater. A little coolant came out and spilled into the container. We cleaned up the threads on the heater side (stainless steel) and the hose connector (brass), then applied thread sealant to both. The heater manufacturer called for Loctite 577 or Bondline thread sealant, but we couldn't find them. We found high strength Loctite and Medium strength Permatex though. After a quick consultation with Geralyn's sister in Iowa, Pam, we decided to use the medium strength over concerns that the high strength thread sealant would create too strong a seal and damage the heater if we ever had to take it apart again. The thread sealant takes 20 minutes to set and 24 hours to cure, so later on this week, we will top up with coolant to replace what we lost when we took the fitting off. Then, with the heater exposed, we are going to crank up the engine at the dock and look for leaks.
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