awww I would say that any form of weathering would be good. The big thing is what the wiles of time do to that whatever that's been sitting out in the elements for who-knows-how-long.
Rust is, after all, only one dominating means of wasting away. If you consider my own project idea... We have a complex mechanical unit comprised largely of metal and probably plastics, that has been sitting on a beach, probably washed up from deeper waters. Looking at the weathering on this would include: Rust rust rust.... breakdown and corrosion of materials, fading and cracking of paint, and damage from wind, sun and rain. The subject will most definitely be at least somewhat buried by the sand and gravel of the beach. On top of that, the ocean provides some other rather unique telltales of time's tolls: Barnacles, mussels, seaweed and such. That sort of stuff, while not a direct result of breakdown, is definitely a factor that contributes to the condition of the modelled subject. Most likely I will want to see if I can show water pooled around it, maybe a splash from the changing tides.
The Space 1999 Eagle shown up above, while not rusting, definitely has signs of age and weathering through it's scruffy, discolored appearance and obvious tellings of lack of maintenance - missing bits and such. I would say that model, while obviously completed prior to this build getting going, should get some sort of "honorary recognition" as it does provide a variation of the "wasting away" theme.
So, I would say there are definitely many forms of "wasting away," including rust, cracking, fading, wind-erosion, dust and sediment build-up, moss/algae/lichen build-ups, etc.
So jump in, show us what ya got in mind and lets run with it?
~Aric Fisher
aric_001@hotmail.com