I wonder if Plasti Dip would work? Not sure what sort of surface prep you would need to get a good bond though.

I've never tried it, but wonder if Artists' gesso wouldn't work - the acrylic based. It's white, but you could color it with acyrlic paint.

Rustoleum has a spray primer (brown) that I often use on metal garden fixtures. Could very well come in a can, too. I still think you'll have to prep them by removing surface rust.

WTS, depending on the metal, the rusted cages could still last a decade. Personally, I kinda like the look.

Why do they have to be rubberized? If it is just an appearance issue just paint them with any color of Rustoleum paint.

Auto paint stores sell a primer paint that kills rust and leaves the treated area black. Old auto restorers use alot of it.

But it all seems an awfully lot of work for cages that will work fine as is.

Dave