A
soldering flux specifically designed for use with
lead-free plumbing solder.
I don't know why it works so well, but it does --
when I first tried to use lead-free solder, with regular flux, it was a royal pain and nearly impossible, but something, and I think it's flux like this, makes a big difference. Partly I guess it's because it contains 8% powdered
solder, which melts early and
tins the pipe and sort of sets a precedent for the rest of the solder (the stuff you'll be applying from a spool) to follow.
(Disclaimer: I have no connection with the Oatey company, and there are probably other lead-free fluxes that work as well, but good ol' No. 95 is what I always seem to find myself with.)