Originally Posted by glostersa
First remove the rust by sanding with a fine emery paper then get a blowtorch and heat the weld apply some soldering flux and once the flux runs freely add some solid plumbers solder to the joint it should flow into the joint. If the gap is wide take some thin stranded copper wire and wrap round the joint and in the joint to act as filler then flux and solder this should effectively reseal the joint and stop any further water intrusion.
thank god there is finally a good answer, i was expecting the usual, jb weld, etc etc
i suggest to the OP that you do some test solders on some scrap metal before you do this if you have never soldered before. now in this kind of situation though how ever much i hate having to say it jbweld would work fine probably ~looks around to make sure fellow students/profs didnt hear me~
now few warnings, make sure to remove all the water out of the section before you solder it or at least make sure it all evaporates before you flow your solder. also heating a closed piece that has a solder joint already will make it blow apart and hurt your self very very bad. my metalsmithing prof did this when she was in school and got stunned and couldnt hear for a while. and it was just a small sphere in the 1/2-1 inch size range