Rusting Iron Filament

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.

Iron filled PLA before rusting

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.

Salt and water bath to rust the print
Salt and water bath to rust the print

After about 13-15 hours and some air drying I achieved the light rusting effect I was going for.

Dragon with light rust
Dragon with light rust

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.

Specialty Filaments – Steelfill and Brassfill

In addition to all the plastic filaments out there (ABS, PLA, PET/G, Nylon, TPU, etc) there are some speciality filaments that mix plastic (often PLA) with another material. The added material can be metal, wood, cork, glow-in-the-dark additive, pine needles, carbon fiber, and even coffee. Today I am looking at my experiences with steelfill and brassfill filaments.

Important Stuff to Know

Metal, carbon fiber, and glow-in-the-dark additives will damage normal brass nozzles by wearing the nozzle opening. This can happen fairly quickly so you will want to get a nozzle that is capable of withstanding this wear. Your options include a hardened steel nozzle or an Olsson Ruby Nozzle.

Also keep in mind these filaments are still mostly plastic. The manufacturers are mixing metal and wood fibers into plastic to create a hybrid filament. Typically the non-plastic element is between 5%-30%. Most manufacturers do not list exactly how much. This means you can’t print in brassfill and expect it to be as functional as true brass. In fact it will still be susceptible to all the degrading factors that PLA has. Brass gives no added heat or UV resistance, no added strength, and rarely gives added conductivity.

These specialty filaments require post processing to have any value in many cases. Metal filled filaments will need polishing, tumbling, or aging. Wood filled filaments will look best when sanded and potentially stained.

The exception here is carbon fiber filled filament which provides added stiffness and strength to regular plastic filaments. For the best mechanical properties I would suggest using a tough plastic filled with carbon fiber like NylonX.

In all cases specialty filaments are more expensive than regular filament without the filler. Sometimes significantly more expensive with rare or expensive fillers.

On to the Brassfill and Steelfill

If you read the above you might be wondering what is the point of certain speciality filaments? With the exception of carbon fiber filled filament, most do not offer any beneficial functional properties. It all comes down to the look and feel for me. The metal adds weight to give it a more realistic feel and it also means you can polish, tumble, or sand to make it look like more like a metal object. I printed this statue with brassfill and one with a non-metal filament that is supposed to look like metal:

Brassfill statue
Brassfill statue after 9hrs in the tumbler
PLA statue
PLA statue with metallic color but no real metal

These were both printed at .15 mm but you can see the one that went through the rock tumbler has smoothed out quite a bit more which I think gives the impression it was printed with more detail. Despite both being shiny, the metal fill has that distinctly genuine metallic shininess that you can come close to with plastic, but not quite perfect.

The above metal fill statue was rolled for 9 hours with stainless steel screws (about 600 small screws of varying sizes). This was a suggestion that Adafruit had for finishing metal fill prints.

Because I used steel screws the brass took on a much darker color. To contrast that, I also tumbled another object in 100% brass screws and the difference is significant:

Brassfill object
Brassfill “Hand of the King” model rolled in brass screws

The “Hand of the King” model picked up the polished brass color and added some shiny flakes of brass to the model. You can see during tumbling the sword tip broke off which I expected. I ended up later filing it to a point again and it looks decent.

You can see what the brassfill looks like before using the rock tumbler:

Unpolished brassfill object
“Hand of the King” unpolished

You can see the polishing not only brought some shine to the object but it also, like the previous print, smoothed out the layer lines to give a more detailed look to the print.

I also experimented with steelfill with a House of Stark coin (there is a bit of a Game of Thrones theme going on here):

Steelfill Object

The polishing in the rock tumbler did help smooth it out a bit and added some shine, but overall I’m not as happy with the result. I think a lot more post work would need to be done to achieve a better “stainless” look. Also I found the weight added by the stainless fill provided less of an impact than the brassfill filaments. Considering how expensive the stainless steel filament was I would be reluctant to buy it again.


  • If you are going to roll your objects in a rock tumbler make sure they are sturdy enough to withstand the forces
  • You can make objects even heavier with more infill but at the cost of print time and filament
  • No matter how much cleaning, even with soap and water, I could never get the polished metal objects to completely stop marking up anything they touched (my hands, paper, cloth) – I ended up spraying them with clear coat and now they can be handled without issue
  • If you want that golden brass look make sure you use 100% brass screws and not plated – Even brand-new brass plated screws will turn prints dark grey
  • Filament with metal fill are more brittle and will be more difficult to print on printers that are not using direct-drive extruders
  • Store specialty filaments in an airtight container to avoid oxidation or water absorption in the case of woodfill or corkfill
  • Tumble prints for a minimum of six hours for small objects
  • Don’t expect true metal properties; you are still dealing with a filament that is mostly plastic
  • Use a nozzle that is resistant to wear for filaments that have harder materials
  • You will get strings and blobs with metal filled filament on more complicated objects, but tumbling typically takes care of these for you