Mild steel is a ferrous metal that will oxidise over time, giving a warm brown patina. I developed a "speed rusting" process that gives a thin, even layer so the rich colour is achieved straightaway, without having to wait a year for the same, much slower, natural weathering. I then seal the surface with Owatrol Oil before delivery.
Owatrol Oil is a versatile, high penetrating, air drying oil. Used as a rust inhibitor it provides a tough, flexible finish, driving out excess moisture, displacing it from rusted metal so stopping rust dead in its tracks, and protects it from further rusting.
It gives the metal a soft sheen, really enhances the colour and leaves the metal clean and dry to the touch.
If you would like to keep the oil topped up then you can buy a sample size pot from the company's website for £7, inc delivery.
It goes a very long way! I use a small piece of sponge to apply a thin layer a couple of times a year.
Ultimately it won't hurt the sculpture if you don't reapply it, so it's up to you.
I use stainless steel rod and cutlery frequently. The silver colour presents a nice contrast to brown mild steel and, as the name suggests, it "stains-less" than mild steel and is very rust resistant.
The majority of the 'dirt' on the sculptures comes from dust in the rain or droppings from our feathered friends, and can be cleaned off with soapy water or wd40 on a cloth.
It is not uncommon for specks of brown to appear due to imperfections in the surface of the cutlery, chemicals in the rain or contamination from the welding process. A good metal polish will clean these off, restorating the original shine. I particularly like solvol autosol as it is formulated to "remove corrosion, tarnish and discolouration...restores and polishes to a brilliant finish...leaves an invisible wax coating to ensure long lasting shine".
It's widely available in Halfords, Wilko, Ebay, Amazon etc, but any metal polish should work fine.
It varies depending on piece and location, but a clean 2 - 3 times a year should be plenty.
I tend to obessess about the cleanliness of my sculptures and love the highly reflective cutlery to sparkle like new, but many people report back that they never clean them and are happy to let them gently weather, so I guess it depends on personal taste and how much you like cleaning!!