Weathering can be anything that makes your plane, or any model for that matter, look old and used. A brand new warbird, for example, has a perfect paint job, shiny metal, no cracks in the tires. One that has sat for a time on the airport flightline, or seen a lot of combat time, is scratched, has oil and grease stains, dents, oxidized areas on the metalwork, dirty, and all these things can be considered "weathering". In relation to wood, weathered wood is more worn, the natural color of the wood has darkened, the grain is more pronounced from being eroded by exposure to the elements.

I see a former as bing more of a skeleton part of a framework, meant to give something a part of it's shape, and a bulkhead as being more of a solid piece that partitions something or supports a very heavy load, as the firewall in an airplane. I think a bulkhead can be a former, but I don't think of a former as being strong enough to serve as a bulkhead.