I’ve just finished rusting my miniature dragon door knocker. Its printed in Proto-Pasta Magnetic Iron and rusted in a salt water solution.
Now to do a full size one 
Cool design by @Sonia_Verdu (just for reference the creator here, in case someone wants to follow 
)
Big thanks to @Sonia_Verdu for the design
@Brian_Wood it was a modified NopHead Mendel90 with an E3Dv6 using a 0.3mm nozzle and a 0.1mm layer height. I printed on an aluminium bed with a PEI sheet attached but most beds would work. Cost will be dependant on where you buy the filament, I think it was less than 5m.
@Brian_Wood its an iron plastic composite, so its iron in the same way that MDF is wood if you see what I mean (basically its iron powder with PLA plastic as a bonding agent). The mix is pretty good though, to the eye and touch its like iron albeit lighter than the real thing.
LOL!