I need to replace two gas tank filler necks that were soft soldered on. For a little extra strength I want to use 2% silver solder on the replacement necks I machined. There are remnants of the soft solder on the tanks and I'm wondering if I can go over those traces of soft solder or if I have to remove every last bit of the old solder. Tanks are new, never had gas in them and the first set of necks were soldered on about a year ago so everything is new, bright metal.
Any thoughts or suggestions are appreciated.