In article ,
says...
Is it necessary to clean the enamel from enamelled wire before
soldering it?

Don't know what they're using these days, but used to
be two kinds called solvar and formvar. IIRC, solvar
is softer and melts away at soldering temperatures
almost acting like flux, while formvar is more durable
and you can burn it off with a lot of heat but you may
not get a good solder joint and chemical or abrasive
cleaning is recommended. If in doubt, clean.