Remove any loose surface rust with a wire brush and/or other means, repair any "real" rust if it affects structural integrity and then just paint with a rust encapsulating paint product, either spray or hand-applied with brush/roller. If the latter, use a really thick nap roller like you would use for a concrete block wall, so it gets into the wire mesh grid and be sure to paint from both sides of the mesh. I'd do the bottom first, personally.
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The most expensive tool is the one you buy "cheaply" and often...