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Comol is a 'not too soft solder' with a 396F melting point so ideal for a small fix on silver soldered work with a little 'hole'. It did the superheater this afternoon so maybe we go with it. One drawback is that once used you cannot go back to silver soldering, it has some muck in it!
Best of luck, David. I'm sure that the truss you have fitted to her underside will stop her belly sagging. Regards, Steve