Proto-Pasta makes some really interesting filament hybrids including copper, brass, steel, and iron. PLA is mixed with metallic dust to create something around 70%-85% plastic and the rest metal dust.
For the iron Proto-Pasta it means you get two added properties over normal PLA.
Magnets are able to stick to iron filament prints
Iron is prone to rusting and you can achieve a rustic or aged effect by purposefully rusting your prints
Prints that allow magnets to stick to them are as easy as simply printing a model with at least 15% infill. The magnet needs to be fairly strong, but does not need to be rare earth type magnet.
To achieve a rusted look for iron prints you need water, salt, and time. You can create a wide variety of rusted looks depending on how long and what parts of the print you expose to the water-salt mixture.
I printed the, now famous in the 3D printing world, Aria the Dragon by Loubie, in the Proto-Pasta iron fill PLA.
Before starting the process of rusting the print I used some steel wool and light sanding to expose more of the iron dust elements in the print. Next I combined warm water and salt in a bowl with the dragon print. I used a paper towel to keep the tops of the print moist and put some lightly wrapped plastic wrap over the whole thing to try and keep it from drying out too quickly.
After about 13-15 hours and some air drying I achieved the light rusting effect I was going for.
The longer you leave the print in the salt water solution the more it will rust. Some people have been able to accelerate the process and even make entirely rusted models with other solutions. Just make sure you take proper precautions when mixing chemicals other than just salt and water.
If you plan to try printing with iron filled PLA be sure to use a hardened nozzle or the Olsson Ruby tipped nozzle. The normal brass nozzle tips will get worn down with metal filled filament.