Sandblast and paint!
Finishing Ironwork for Outdoors
Sandblasting and painting blasphemy? How about 1/8" deep craters in the surface of your work in 5 to 10 years? Thin stems and leaves disappearing in 20? How about rust stains running across your customers "like new" concrete work and bad mouthing you for doing lousy work.
Not only is paint the ONLY suitable protection for outdoor iron work (in most climates), but there is only one system of applying paint that really holds up.
First, the iron must be clean. Sand blasted or acid etched nothing less. If cleaned with acid, the acid must be neutralized and THAT rinsed and possibly passivated afterwards. I prefer sandblasting.
Second, the first coat must be cold galvanizing, a zinc powder paint. NO this is NOT zinc chromate or so called "zinc rich" paint. ALL of those are rip offs and a waste of time. Cold galvanizing is 99% zinc and a little binder. It is available in spray cans or in bulk. It is what is used INSIDE water tower tanks!
Third, a suitable neutral ph primer. I use Dupont red-oxide sanding sealer. This isolates the chemicals in the pigments from the zinc AND the iron.
Fourth and last, a top coat of almost any exterior paint, lacquer or enamel, automotive finishes are generally the most color fast.
This finish will last 20 years or more and will require repainting only for cosmetic reasons (dirt, moss, mold).
I know a LOT of smiths that do gorgeous work but leave it to the customer to throw a coat of paint on the work. Sure, then the customer is responsible, but its your work turning to dust! Your work AND your customer deserve better.