[drolston]

Faced with an $800 re-core, I painted a thick coat of liquid FlexSeal on some miniscule leaks (more like weeping) on my radiator. It has held up for about 1000 miles on some hot days and with a 7 lb radiator cap. I bronze brushed the adjacent surface nice and clean.

So, if you can get to the leak with a little brush, it might be worth a try.

Or, Barr's StopLeak is pretty good on pinhole leaks.