I plan to have them galvanized and polyester powdercoated. I would appreciate any advice on these issues. I would also appreciate recommendations on galvanizing and powdercoating shops in the San Francisco Bay Area. You should also consider Enameling, or porcelin coating these things. It would be much more expensive, but the resulting coating would be WAY more likely to last your twenty years without failure.
Steel can be flame sprayed with zinc or even aluminium.
The alternative is true hot dip galvanising of the welded assemblies. Zn should be more than adequate for longtern durability. Jeff - I'll look into the enameling. Roger - Thank you very much for the information. I'm not familiar with those finishes.