[aurizon]

I used to work for Schenectady Chemicals in 1968 we developed solderable self fluxing polyurethane coated enamelled wire, it was an immediate hit and soldered well. Times have changed and I left them in 1978, but it might be an item to look for as I found it very handy.

[burnt-resistor]

Possibly vaguely similar product from a German manufacturer: https://www.sh-wire.de/en/applications/product-range/shsoldr...

[aurizon]

Yes, urethane decomposes liberating longer chain organic acids which fluxes