Scrape the oxide off before starting. Multicore should work, as would the
plumbers solder and plumbers flux paste. I wouldn't use offcuts though as you
would be welding rather than soldering! Plumbers solder will be "lead free" so
have a higher melting point than multicore. I think Bakers' fluid (Zinc
chloride solution with a bit of glycerine & alcohol?) will work but less
convenient than pastes?