In the 'old days', lol, we'ed just stuff a piece of potato into the tube opening. Smaller leaks fixed with a tablespoon of coarse pepper, and/or one egg white. However, these days you must decide other measures now. So far, the preceeding replies seem good enough. I'd consider in addition.., to first clear away some heat sink from the problem hole.., and 'pinch' the open tube hole with a needle nose pliers. Not all the way through, (thereby shutting off any flow through the tube), but just enough to permantly shut the edges of the hole down to be closed flatly, and tightly. Thus you have only restricted 1/4 up to 3/4 of the tube still allowing tube flow. Then use some epoxy system as already described, on that dried out spot/area. LOL, people look at my many 'patched' areas of my OEM radiator, and are somewhat shocked. If you go to any marine store, you should also find an epoxy that likes water, i.e., can be used around wet holes, i.e., quickly fixing a 'sprouted leak' such as a radiator on the side of a road. Maybe someday I'll get around to buying a new rad.