Old cattle trough as a planter
So, I've come across an old cattle trough, I'm pretty certain it is galvanised steel, but it is rusting quite significantly (see pic). There is also some patches of white solid encrusting the inner walls which I presume is calcium deposits. It is still strong enough - I've drilled several holes in the base so I've got a feel for its thickness. I intended to plant some edibles in there but I keep having thoughts (dangerous!). From what I can read, rust and Zinc wouldn't be an issue - the amount that will leach into the compost will be comparatively little to the RDA, and the plants will only take up what they need anyway. Do correct me if anyone has an opposing opinion though.
I was thinking of trying to make the interior rust-proof, either by applying a Zinc cold-galvanising spray, or by painting with a rust prevention paint. I've been recommended Hammerite as many of their products can be painted directly onto rust. But I keep wondering whether the chemicals in such products would actually be harmful to both plants and anyone consuming the edibles grown within the trough. If that's the case, I'd prefer to leave out the rust preventative and go naked, so to speak.
I've tried looking up this information online and haven't really got anywhere beyond rust and Zinc are fine. Does anyone have any thoughts?
Cheers in advance.