I'm not a builder, but I see a lot of older bikes at my shop and saw a few good posts on this before the framefo went down. This is an imaginary problem for most people. Oxidation is actually self limiting to a degree. It forms a barrier preventing deeper damage. For instance, I've got a spot where a local welder took off the paint on my Surly Steamroller to put on rear brake stops. I've been meaning to paint it for almost a year. In the meantime, it's just been bare chromo. I've ridden in the rain a few times, had it out tons... and there's only a bit of discoloration on the very surface. I'll paint it eventually, but it'd take a LONG time for it to be a real issue.

Internal stuff in a large part depends on the care and construction of the bike. Make sure to use plenty of grease on your seat-post if you're going to spend a lot of time in the wet, it acts as a bit of a gasket to keep water from getting in that way. Ditto on water bottle bolts. Don't leave the bike out in the rain longer than necessary, give it a quick dry when you get it in. Some builders make a vent at the BB, so that water that sneaks in can get out again, that helps too.

The most common case I've heard of really deep structural rust is when some paper or cloth gets left in the frame, either cleaning or by the builder. I heard one story of someone finding a rusted through seat-tube with an italian newspaper stuffed in it... that sort of thing holds liquid in contact with the tube and prevents quick drainage or evaporation.

I'm not trying to say that stuff doesn't rust, just that if it was the pervasive problem that a lot of people worry it is, we wouldn't have all of those 70's era road bikes found in people's back yards still rolling around.

If you're paranoid (or live by the water, or in another area with salty air or corrosive dust), use some frame-saver... most bike shops can order it. It's a rust-arresting and preventative spray that you can roll around in your frame to guard the inside. Some intelligent builders call even that level of preventative maintenance unnecessary. You can cover external scratches with clear nailpolish.

Also, never bring a bike you don't want to die to burning-man.