Yes I'm using flux-core wire and I do indeed have the polarity set for flux-core.
My conception was that we're adding power by increasing the voltage and we're stacking up material by adding wire, so to get good penetration on thick material I want lots of voltage and not much wire feed.
But actually it turns out that you make it hotter by increasing the wire feed speed.
I don't really see why, but at least now I know.
Here's a more recent weld: https://img.incoherency.co.uk/6464 - still not great but not nearly as bad.
Nice weld, and nice project!
I'm learning with you.. the polarity was fresh in my mind because I recently got half way through a project with little bb's splattering everywhere before a friend with more knowledge asked if I had recently changed wire.
Welding seems to be a lot like baking; It is very deterministic in a sealed environment and the parameters are well understood, but in practice the experts rely heavily on feel and experience more akin to an artist.