Painting of corrugated iron catchment tank

[JWFITZ]

A good solution and one used most often on ships and maritime metal is Ameron 235 "barrust" paint. US Navy option 1 for rust control, and it's rated for 10 year immersion. You can buy it at Big Island Marine, and it isn't expensive. Use that for a primer base coat and a cheap enamel like rustoleum for a top coat, touching up when needed. It's a real fix.