Spotted this over in the comments section of a pinshape blog about aging and polishing metal filament 3D-prints, and it’s worthy of its own blog post imho!
Great topic thanks! Just as a side tip, if you mix 1 part white wine vinegar, 1 part peroxide and then enough salt so it doesn’t dissolve any more, Just brush it on and it will start rusting iron within seconds. Just make sure to give it time to dry or you’ll rub or wash all the rust off! I tried a spray bottle as well but the salt gummed it up.
Here’s a part I printed that fully rusted to this in just a few minutes. Once dry, I gave it some love with rough steel wool and then sprayed a flat mod podge sealer coat on it to keep the rust from getting on my hands.
Attribution to the owner of the Thing I printed