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You could try some stop leak. I personally would not use any it may gum up the cooling the system.. As far as a fix you should be able to remove the radiator and take it to a repair shop and have the gasket on the side tank replaced.
I had a pinhole that later developed into a crack. Stop leak didn't work, I simply replaced the radiator.
I did drive it for a couple weeks like that with it squirting coolant but it didn't harm anything.
I used some putty-like compound from the hardware store that was rated to 500 degrees F. It was like JB Weld. I put that on a rather large leak on a 1988 Oldsmobile (3800) radiator. Worked so well taht I forgot to replace the radiator!!! I sold the car with 320,000 miles on it(original drive train... and I absolutely PROMISE that I am not making this up. The car was a freak and is probably still going). Go figure.
It's a great fix if you're stuck but I would not put anything
thick like an egg in my rad unless it was life or death.. May
cause problems down the road ( no pun intended ). I would
take eschoendorff's advice... Keep the fix on the outside...
The radiators are cheap-as little as $130 and easy to install, mine took less than an hour for removal and installation. C4 radiators are some of the easiest to put in.
A patched radiator is a ticking time bomb............