Hi Sidney, Normal silver solder will “take” on steel. Plating works
best on a chemically clean surface. I should think a quick dip in
muriatic acid would do. Very quick: you don’t want to dissolve away
the piano wire. The thinner the wire the greaterthe risk. Then rinse,
ideally in distilled water. Do this ventilated or outdoors; muriatic
fumes in the workshop will rust your tools.
Soldering heat will soften the piano wire. If you are using it for
its springyness you have a problem. You may be able to quench harden
it by dumping the assembly into water all in one swift motion
immediately after you take the torch away, while it’s still almost at
soldering temperature. But that is pretty ad-hoc. It may work on some
pieces and not on others; it may also make the wire not only springy
again but brittle in addition.