This is a ridiculously simple effect to add to any cast iron faux finish project! I did this some time ago so I don't have photos of all the steps, but all you really need to see is the final result. Here are the steps I took photos of...
Step 1: Build a Structure
I love old vintage rusty things, so I decided to build a small shelf for my desk. I'm always collecting what i call "cardboard treasures" that come as packing braces with products I get in the mail, so I had a stash of sturdy cardboard pieces to build with. In order to sell the cast iron look, I drilled holes in the legs and built it as if I were using actual iron members. Cardboard is easy to work with and you can build a sturdy structure in no time using a good quality wood glue and sound structural techniques.
Step 2: Add Hot Glue Welds
This is the effect I think best sells the final look! Simply lay down small beads of hot glue in the areas of the structure that would typically receive a weld. These are not for structural integrity at all and only add to the final appearance.
Step 3: Faux Finish
There are many "rusty iron" faux finish techniques out there, but my favorite one is simply black paint, cinnamon, and a bit of metallic paint. Here's a good tutorial:
I ended up using straight black acrylic paint, adding the cinnamon while it was wet, wiping it off as needed, then spot painting in certain areas with some craft metallic paint i had laying around. Lots of ways to get this final effect.
The only thing is that when it's all dry, the cinnamon smell is strong and you'll get cinnamon on your hands if the item is something that's handled often (like in my case). So I gave it two coats of Modern Master's Dead Flat Varnish (which is an amazing product that seals in anything that's actually rusty so that it can be handled and used safely around the house) to seal it and make it ready for use.
Step 4: Show Off to Your Friends!
Everyone who sees my shelf is jealous about my vintage antique find, and is then amazed that this is made out of cardboard! But beware, this workflow is addicting, and now I basically want to 'ironize' a bunch of things around the house. Enjoy!