Painting exterior on clawfoot bathtub
I need advice on painting the exterior of a clawfoot bathtub. The exterior of the tub was blasted with crushed glass (soda blasted on the interior) and all of the old paint was removed with this process. The cast iron has some pitting, probably due to age and years of sitting outdoors, but the blasting cleaned it up nicely.
The same day, we rubbed it down with denatured alcohol and then painted it. First we sprayed it was two coats of Rust-o-leum Professional Primer. We were careful to follow all of the manufacturer's instructions and the weather conditions for spraying (the tub was outside on a trailer) were within the parameters specified. We then sprayed the tub with two coats of Rust-o-leum Hammered metal paint. We picked this because the color and the textured would look the best of any of the Rust-o-leum paints available. Again we were careful to follow the manufacturers instructions. This was done about 1 1/2 week ago.
The tub has remained outside and it has been rained on a few times since it was painted. We now starting to see rust showing through where there is pitting along the sides. The top coat of paint has not hardened to the point where it cannot be scratched off with a fingernail.
We plan to strip the paint and redo it, but this time I want to be sure that I'm using a paint which will not allow the metal to rust. Eventually the tub will be moved into the house, but since a bathroom is a moist environment I need to be sure that the paint can withstand the humidity. I know I could spend a bunch of money and have the whole tub urethaned by professionals, but the whole point of using this tub was to save money by doing the project ourselves. I'm not concerned about the interior of the tub; it's got some stain but it's fully serviceable as it is. I am however concerned about the exterior. I want a good durable paint that will last for years to come.
Any suggestions? Thank you in advance for your replies.