Not sure it would work as a bird bath they were designed as feeders for cattle. I'd have though the water would end up polluted from the iron and not good for the birds. Perhaps you can just use it as a very fancy planter?

Wellit should be alright to use as a bird bath if you put enough layers of Hammerite on it - have a look at their 'direct to rust' range, which they say can be applied to rusty surfaces without undercoat. Comes as a spray or in a can to apply with a brush.

I agree with Mg. It is not suitable for a bird bath because it rusts. Hammerite is HIGHLY toxic to birds and should not be used on anything animals come into contact with. There are specialty paints available for that use.

There are several rust converter products for serious stuff. I used Curust very successfully to restore some very rusty parts in my old VW Beetle with it - took several coats but ended up rock solid.
You can then paint over the result with any suitable paint.

If you look up Rust Inhibitor Paint on google you'll find several different ones. If you contact the manufacturers they should be able to tell you whether the result is inert but if you paint over it anyway it wouldn't matter. I think you'll find it cheaper than Hammerite for a biggish job - its used to repair rusty farm machinery.

In light of some scary statements in this thread, my curiosity was piqued, so I contacted the manufacturers of Hammerite products yesterday - they've replied via email today and they say that Hammerite is not toxic to birds provided the paint is allowed to cure for 7 whole days so that its completely set hard prior to allowing a bird near it. After that time, its non toxic. However, they also said that Hammerite would not be suitable for the interior of the pig trough if you are going to keep water in it, so you can do the outside with it with no toxicity issues, but you can't paint the inside, I'm afraid, simply because Hammerite won't stand up well to being permanently immersed in water.

Hammerite paint will continue to emit low-level toxins into the environment for years after application. Birds are ultra sensitive and delicate creatures. Why tempt fate?

There are other paints available that are much safter for birds such as; SoyGuard, EcoDesign's BioShield, Unearthed Paints, etc. You'll have to do your own research to find one most suitable for ironwork.

Try Toadsnatcher.com. Steve Dudley has one and works for BOU.

Why not just use it as a very attractive planter which at least one company that sells them suggests?

That's a good idea. Any shallow dish would work - many many choices at the garden center or craft store. Save the Hammerite for your lawn furniture or mailbox post.