While there's no way to "prove" this, and it's certainly debatable, I'm confident that a GOOD patch job on a radiator that is in decent shape to begin with should last indefinitely.
By GOOD I mean
- Clean the area with window cleaner, then again with isopropyl alcohol.
- Scuff the area with 60-80 grit sand paper, just deep enough to get good adhesion, but not so deep to remove excess material.
- Clean the area again with isopropyl.
- Clean again with isopropyl (seriously, the cleaner it is, the better adhesion you'll get).
- Properly and thoroughly mix JB Weld (tested by Project Farm as the best overall 2-part epoxy)
- Apply JB Weld liberally, working it into every crevasse.
- While JB Weld is still wet, push a layer of fiber glass mat onto it, making sure the JB Weld soaks in uniformly.
- Let the JB Weld tack up (5-6 hours or so) and apply another layer or JB Weld and fiberglass.
- Anything more than three layers of JB Weld/fiberglass is probably overkill. You're more likely to break adhesion to the radiator and develop a leak through the gap than you are to develop a crack in 3 layers of JB/fiberglass. This is why scuffing and cleaning the area is so important.
If used properly, for the right application, JB Weld is pretty amazing stuff. This is a pretty ideal use case (ie patching hard, non-flexible plastic). When combined with fiberglass to increase tensile strength, it should hold indefinitely, especially if a significant crack hasn't formed yet (plastic will have a much harder time cracking open if it's being held together by a strong patch). Also, you might lose visual indication of a failure (or not), but if it doesn't fail in the first place, that's a non-issue. Also, the radiator itself is around $300-$400, plus shipping/tax, plus installation, it's closer to $800-$1000 total. JB Weld and some fiberglass is more like $8-$10. That's the route I'm going personally. Time will tell how it holds up. I'll post updates here how things are going over the years.
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