Many years ago, while in college, I was driving my Triunph TR3 on a long, highway trip. It was very early in the morning (about one, or two) and with only about a 100, or so, miles to my destination, I pulled over to the side of the highway and caught some shut-eye. When the sun came up a few hours later, I felt refreshed and excited to complete the journey. I started the TR and lit it up in an aggressive departure from my rest stop. Unfortunately, I was a bit too aggressive and the engine broke it’s engine mounts and leapt forward and inch, or so, causing the fan blade to take out the bottom two inches of the radiator. With water flowing everywhere, I immediately pulled over to take an accounting of my situation. I was literally in the middle of nowhere (rural Illinois farm country and the nearest house looked to be a mile away) and I was stuck. I had to get creative. I pulled out my tools and yanked out the radiator—-it looked pretty bad. Then, I remembered. I had a gallon can of Bondo in the trunk of the TR. I pulled it out, mixed up a big batch, and packed the lower half of the radiator with half-a gallon of Bondo. I let the unit dry for a couple of hours and put the whole thing back together and filled up the radiator with water using a discarded soda can and water from a nearby runoff trench.

I still own a TR3 (not the same one), but when I sold my first TR3, several years later, it still had that Bondo’d radiator in it and it ran, under pressure, as cool and as leak-free as if it were the original unit. I like to look back on that incident with a feeling of accomplishment, and fondly remember it as one of my finest hours.

So, Chuck, I encourage you to pursue your creative urge and reach for your “finest hour.”

Good luck.

You should as the other responder said, replace the radiator. If you want to try repairing it, I would try a 2-part SLOW-DRYING epoxy. The quick-drying ones are not water or chemical resistant - it says so on the package. If the hole is of any size the epoxy will fill it without the vacuum. You will need some kind of backing or mechanical support behind the glue - otherwise the system pressure will blow the patch out. When mixing the epoxy, be accurate about the 1:1 ratio - do it by weight if you have a precision scale. This may not work because many adhesives, including epoxies, don't stick well to plastics. Also no adhesive will work for long if the joint is able to flex at all, even microscopically.

I don't know if that would work, but it's ultra creative. It might be tough to find an adhesive/sealant that's competent to handle the combined challenges of that applicaiton. Thermal expansion, exposure to hot water and coolant, etc.

Your idea sort of reminds of a friend who years ago found himself with a severely cracked IH Scout radiator tank in the middle of the desert and repaired it by heating the tank in a campfire and using a melted pot-metal license plate frame to solder the radiator. This worked well enough to get him home alive.

You'll just be wasting your time. It leaks because the seam seal is NG and (the last I've heard), no radiator shops even try to fix leaking plastic tank seals.
It's just time to buy a new radiator, not unusual for ANY older car. Sooner or later, all cars need new radiators.