Rusting Perfectly Good Metal

Back in the day I remember seeing old buildings with metal roofs that were rusting and thinking how cool they looked. There was something about them that reflected an aura of gentle aging, of the graceful passing of time. Today, with a little help from chemical science, we can speed things along and have that 'aged in time' look in an afternoon. Corten was the first 'rusting' metal alloy that I remember and was quickly accepted as the material of choice for guard rails on highways throughout the nation. The logic of the Corten metal is to combine dissimilar metals into an alloy that produces a kind of self sealing finish, a rust that stops rusting.  A stabilized rust. It took a few years but that same technology has moved into the construction mainstream. The relatively high cost of this type of metal has helped to popularize cold rolled steel as a viable 'rusty' alternative. Allowed to weather in nature (or 'rushed' with the use of added corrosive materials) it produces a more organic patina than the very uniform look of Corten type alloys. The natural look of rusted cold rolled steel and it's greater cost effectiveness made our choice easy. Because of our climate (dry) and the wall wainscot installation (dry) we needed to speed Mother Nature along by pre-rusting the raw metal before installing it on the building.

Here comes the 'smarter than' part. You would think that just spreading on some muratic acid or vinegar would produce the desired effect. Wrong. Again, we are in a hurry here, time is money and all that. The process of cold rolling steel is itself an issue. The pressure necessary to produce the product seals the metal surface from the quick action we would hope for. Add to that the small amount of oil that is used to keep the cold rolling process running smoothly and you have another layer of protection against the immediate 'ravages of time' effect we are trying to produce.



New, untreated corrugated steel

In the past I have used a biodegradable but industrial strength soap to hurry along the etching effects we look for on galvanized metal, but with this material it seemed relatively ineffective. I had heard from others that sand was a key to help abrade the surface to receive the corrosive effects of the acid.  First I sprayed the surface with diluted muratic acid, then sprinkled on some sand, and with a medium soft floor broom proceeded to rough up the metal surface. This worked fairly well but I still had to spray on a second coat of acid and do more brushing to get anywhere close to the surface patina I was looking for.  

Prepped with Acid & Sand

I next tried to use salt instead of sand and/or in conjunction with sand to speed things up. This worked fairly well but the early patina produced by this mix was not particularly attractive. Still not satisfied with the effectiveness of my attempts at instant rust, I went against my stubborn maleness and asked somebody who knows. Barry Stromberger of "The Slag Works" has been working in metal and metal patina for most of his not insignificant years. He had the answer, ferric nitrate: "Just mix with water in the correct proportions  and (after the initial roughening of the surface), spray it on and watch it go".

Perfectly Rusted Wainscot

 I love this stuff. If you have properly prepared the surface, with little effort you can  produce a gorgeous and natural looking rust patina that will range in color from a kind of yellow gold (short duration) through a very organic red, all the way to chocolate brown (just let it sit). Once you are satisfied with the rust tone, rinsing with water stops the action, or close enough. The reality of cold rolled steel is that in a wet environment you will have continued rusting. In our exterior wainscot  installation and semi arid climate this should not be a problem. The galvanized corrugated roofing we have used in the past is typically 26 gauge whereas the steel corrugated material is 22 gauge, much heavier to withstand the the reality of its use.

                         This is one time where 'instant gratification' has paid off handsomely.