Sealing a leak in a cast iron radiator with automotive radiator leak sealer?

Boilershepherd

A new customer asked me today this question, ' Can we use an automotive radiator sealant to fix this radiator?'. The specimen is a cast iron sectional with a very small leak coming from somewhere within or between the sections. the system is a small Slant/Fin Sentry 120,000 BTU. No zone valves and a single Taco circulator on the return.
I suggested that we not proceed this way but it made me curious.
Otherwise, are there methods to heal radiators in order to keep in service?

Jamie Hall

Oh yes. And automotive leak sealer isn't one that I'd try. Being hot water, it's a bit harder than steam, since the pressures are so much higher, but it can be done.

The best and most permanent technique is to split the sections at the leaking point and replace the nipples between the sections. They are a machined fit, and assuming the seating surfaces in the sections themselves are OK, that works beautifully. If there is corrosion to the seating surfaces, if it's very minor try pipe dope when reseating -- but that's a bit of a kludge. Otherwise, when you split the sections you will see that there is a slightly recessed flat area around the openings. Replace the nipples anyway, but then before you put it back together put a good bead of red RTV gasket sealant all around that flat recessed area. That should hold it.

Sometimes if you can get the metal surface really clean -- no paint, no rust, no grease, the usual -- JB Weld will hold well enough to at least get you through the heating season.

Boilershepherd

Thank you.

Pumpguy

One product that's worth looking into for this application is Loctite's wicking sealant #290.

While sold primarily for use in locking assembled threaded fasteners, it is also recommended for sealing porosity in welded assemblies and metal castings.

Loctite recommends moderate heating and cleaning of the porous area. I've found the heating isn't really necessary; just accelerates the curing.

Intplm.

I have tried using different types of boiler seal directly into the radiator.
Not its intended use but I have tried it. Sometimes it works, sometimes it doesn't.
It's a guessing game as to how much to use or not use.
To little and it will not fix the leak. To much and you can ruin the radiator by closing off the path of steam.
I don't recommend it as a cure fix all type of thing.
(For that matter I don't care much for boiler seal either)
But! It's worth a try.
Maybe the best thing is to find a used replacement radiator. That is what I tell folks when this subject comes up.