I would be very wary of taking it down if badly corroded you may find yourself with a pile of bits. If it is still reasonably sound and not full of holes with pieces breaking off, I would paint it in situ, bitumen paint inside wire brushing treating the outside with a rust converting paint before applying a decent zinc based primer after which a decent undercoat and gloss to finish off. When Hammerite was still good I recall the smooth version was not recommended for exterior use, I don't know if that applies to the current version. Generally Hammerite in my opinion is not what it used to be.

Richard