Assuming that metal isn't so thin as to immediately fall through, I'd save the money. Just spray whatever wd40 type product you have lying about before you cover it up.
The one the LL's builders left in our garden at uni was in worse state than that and it was still perfectly serviceable as a rotating fire pit in the summer.
Yes, the best thing to coat it with on the outside could well be old engine oil, it is what we throw round our old tractor trailers to keep the rust at bay.
Assuming that metal isn't so thin as to immediately fall through, I'd save the money. Just spray whatever wd40 type product you have lying about before you cover it up.
The one the LL's builders left in our garden at uni was in worse state than that and it was still perfectly serviceable as a rotating fire pit in the summer.