I've tried solder in the past with no success, the pressure just blew the solder right back off. Could have been down to me doing it wrong.
Whenever I have had a radiator problem I have found that replacing the thing solves long term problems. But thats just me.
solder will not work.
you'll need to get it brazed, sort of low temp welding. had rads sorted before and usually only cost about 20 quid max to get it done