I had some 26ga rosin core, with a tad of silver left over from my career, Labels are long gone, but it's cored with rosin... Key item is a teeny point, and new tip. don't forget to ground the tip! on my wellers, I loosen one of the screws and attach a wire... If your tip is gonna be too hot, wrap a few turns of 12/14 ga solid copper wire around it tightly.
I was confused on my iron, after checking. I didn't use the WP18... used a WP20, since it had the ground already rigged on it.. (I'm not talking about the newer WPS series, my WP series irons are 25-35 yrs old.)
I think Don used a different flux.
Several use temp controlled stations...