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one of my steam radiators has a leak on the bottom, away from the valve (has been checked, my husband has changed out the valve). can I use a spray sealant to seal this leak? (like FlexSeal)

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From reading on the Internet, FlexSeal looks like a scam ($59.70 for shipping, signs you up for $29/month "membership" without asking). It looks like it's similar to automotive rubber undercoating, which you can buy locally for ~$5 per can.

Don't know if it would work for your purpose. But I can recommend J-B Weld. Sand and clean with acetone, then apply a thin (1 mm) layer of J-B Weld. See also the answers to this question.

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Typically, for anything under pressure, you have to seal it from the inside so the pressure drives the sealant into the porosity or hole. The sealant also has to adhere and also flex under temperature expansion. Marine-Tex and J-B Weld probably are the only sealants you'll have success with on external application. I've used Marine-Tex on engine block cracks, J-B Weld is formulated for similar use. As Vebjorn says, sand it down till you find metal, clean it of all oil and apply. – Fiasco Labs Jan 6 '13 at 3:43

drill a hole in the leak, tap it pipe thread, install dope with pipe plug

or weld or braze or solder

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This would be the best, there are radiator restoration businesses that could clean the leak area and hit it with a bit of silver solder (high temp) and end the problem permanently. It requires someone with experience heating cast iron. – Fiasco Labs Jan 6 '13 at 3:47

I found a small hole in our steam radiator when we were refinishing it. I ended up using Loctite Steel Stick to fill it in. It has lasted a full season and I believe that it will last for the life of the radiator.

From the website:

Metal Magic Steel Stick(TM), Steel Filled, in Stick Form, 4 oz. Stick, Stick, Gray, 10 min. Functional Cure, Shear Strength 2500 psi, Temp. Range Up to 250 Degrees F, Work Life 3 min., Seals Pipes, Gas Tanks, Water Tanks and Vessels, Includes 4-oz. Stick

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