When you listed the materials you omitted copper, used in gutters, certainly should be durable outside. I have seen some aluminum pieces outside, should be durable, but realize that it will oxidize.

Ries Niemi is the guru of electropolished stainless, he frequently does very large pieces that way. He has a local shop with a very large tank for that process, I think that the shop usually does marine work.

I spoke to an artist doing stainless with the swirl finish, outdoor pieces around 6 foot high, mostly. He said that the swirl finish was super-easy to touch up, one of the reasons that he liked it. He used a sanding disk on an angle grinder.

I recently did a swirl pattern on copper, used 220 grit on a 3 inch disk on an angle grinder. (If it had been a bigger piece I would have used a larger diameter disk). Very easy to do, even though it was my first time on that process. When I had that piece in Open Studios, many people, including my spouse, commented that they liked that finish very much. (Note that this is an indoor piece not subject to moisture).

Richard

http://www.fergusonsculpture.com
Sculptures in copper and other metals

Jake,

Well, I actually have gotten very tired of the sander-swirl finish on stainless.  It seems that everybody just automatically uses it since it is easy and very forgiving of poor technique, and hides surface imperfections pretty well.   Sort of the way the "hammered" finish was overused on Arts and Crafts copper and steel pieces over the years.  I've seen too many pieces that would have been much nicer with another finish, but the swirl was used out of apparent laziness or unawareness of other, better finishes. 

You can heat stainless and get a very durable deep gray/black finish similar to steel.  In normal environments it will be stable and not rust, though in a marine environment it may begin to rust some over time.  If you avoid dthe use of steel tools or contaminated grinders and such when working it, it will reduce the rusting tendency.   I think it makes a very nice finish for forged pieces with some surface detail, as you can highlight surfaces with selective sanding/polishing.

As noted, Ries Niemi is very knowledgeable about electorpolishing of stainless for sculpture work.  One side benefit of the electropolishing is that it passivates the surface so rust is much less likely to happen down the road.  It is important to note that electropolishing won't relieve you of the chore of cleaning up your welds, etc.  Like plating, it won't turn a sow's ear into a silk purse, but if your work is clean, electropolishing it will sure make it look shiny and "finished."

I like silicon bronze for exterior work, ,though it is ridiculously expensive.  Copper is nice, but rather soft.  Nickel is neat, but again is really expensive.  Aluminum requires anodizing to have a decently stable surface, which can get expensive, but without it is very prone to damage.

Stainless can be reliably used with a wide range of maintenance free finishes ... I think I have posted a few entries about this here, and there are also some blog examples on my site.

If it is marine, always electropolish and then play with careful oxidation afterwards

Anywhere else, heat colouring with or without the help of acids will yield anything from a stable rust appearance to vivid golds and blues. If you want matt or satin stick to decontamination with acids .... if you prefer reflective and vivid then electropolish before heating.

I have produced thousands of meters of forged stainless over the years and it has never let me down.

Where to bend next ... www.metalgarden.ca