That looks a little rusty. Where is it exactly coming from? Is it a hole or just rusty in general? the core part can be soldered. The tops if they rust out are pretty much done and your better off replacing.
Most radiator shops can take the tops off clean it up and then solder it back up.
However the results are not always great. I've just had the rad in the hj60 done and after putting it back in they started to leak again! Not really the shops fault it is 25yrs old and the original rad. But it only cost me $30 so im not too fussed.
those seams are a bitch to fix however if you insist the only real way i would attempt it is get a can of mapp gas(just like the propane bottle but its a yellow canister and it burns hotter) and find a 4 foot brass rod with the flux already molded on.............they are white.
ok doh k...
that little seperation on me radiator just decided to go growl!!!!
it opened up enough to nearly get my pinky finger in there..
its in the shop right now getting first aid..
i`ll give a pic of it out of surgery 2moro