Unexpected challenge

We were looking for salvage items to use as planters in the refurb garden. Hefty boxes, discarded chimney pots, that sort of thing, and we saw what we took for a rusty old saucepan. We can stick some flowers in that, we thought.

And yet, and yet. First it weighs a ton, and you didn't put that much iron ore into an ordinary pan. Second it comes from the Falkirk Iron Company's Castlelaurie Ironworks which gives it a history. Third, it's a 2 gallon measure, not an old saucepan. So now we're contemplating conservation technique, how best to cut down the rust while keeping the "pateeena". And what was its original use.

I've seen WD40 suggested as a surface treatment. Your thoughts?