When to paint?

[jps]

I'm trying to figure out what order to do stuff in. As my chassis is currently mostly constructed, but sitting in a damp garage, i thought that getting some paint on it would be a priority. But it seems like I need to do all the welding necessary before I can paint - so I need to have at least trial fitted the engine and gearbox in order to know where to attach engine and gearbox mounts. Which seems like a big leap forward from where I am now.

Is painting the chassis actually a job for much later in the build? If so - how do I best protect the bare steel tube/box section until then?