Leave it to a pro- - - -there's a fine balance between just enough heat to make the solder stick, and a bit too much that will turn your slightly damaged radiator into expensive junk! A natural gas/Oxygen torch with a very fine-tipped flame is just about a must, also. Propane/air is too cool, and Acetyline is WAY too hot! There's also the problem with just enough flux and/or acid, applied in the right place, to make the solder flow and seal. It only took me about six months, and a bunch of ruined radiators, to get the technique down pat. Fortunately, the guy who was mentoring me was very patient! He had me practicing on junk radiators in the first place, so I didn't flush his profit margin down the toilet! That guy was an artist with a torch!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!