Andrew R Stewart 

If you have the lug w/ tube in your hand, free of the rest of the frame, you stop grinding out the tube when a new one fits in. A bit less easy to judge if the rest of the frame is still attached but still doable. Some say you will see the brass layer that was between the tube and lug as the tube is removed bit by bit. Applying some Navel Jelly to enhance the contrast of the steel and brass might help.

As to the rust- I wonder if the bare frame got a phosphate etching wash just before the primer was done. Good advice to do this on the repaint either way. I question whether leaving paint on the HT and Bb faces will do much for rust prevention. The paint will usually get squished and "squeezed off" the face with the pressure of the cup contact. What I find more important is to make sure any paint squishing doesn't travel around the corner of the face and onto the body of the HT/BB shell. To prevent this I'll take a small and fine file and cut through the paint around the edge, holding the file at a 45* angle to both the face and the outside diameter. Then I'll file off the paint on the face. I'll add some grease to the face during part installs.

But rust is truly a cancer and very hard to totally prevent given moisture and salts. Andy