If I understand your question, weathering is what the weather does to the wood.. darkens it, colors it, changes it due to weather changes and environmental effects.

I think the term bulkhead and former can be used in the same way. The former, 'forms' the shape of the fuselage. If you see bulkhead in the same format, then I would assume that you can use it in the same manner. A firewall, for instance can be a former, but it is the one that is immediately behind the engine so it's a 'firewall'. It comes from the term to potect the rest of the plane from fires in the engine compartment.