I used a Lyman T Mag, now a Redding T-7 to size all my rifle brass. Because the quantity of 223, 308 and 30-06 ammunition I fired in NRA competition, I left these three dies in the turret heads, assuming that because of the frequency of use, they would not rust.
I figured the coating of RCBS water soluble lube left inside the die after sizing cases would protect the die from rusting. It did not work out that way. All of the dies have internal rust on the bottom 1/4" of the sizing die. I only discovered this when cleaning one die, noticed the bottom was dark, and upon inspection, the surface was pitted. Whatever rust occurred resulted in a pitted surface. Similar to the surface on this axle:
This is going to be a public service announcement, but the pitting on the driver's side axle was directly due to an axle seal failure. The seal leaked, but not "badly", but it leaked. This is the bearing
The passenger side axle, the seal was good, and for an axle that was rolling at the time of the Vietnam War, it was doing pretty good. I believe oil starvation created metal to metal contact on the drivers side and caused the severe pitting observable on the driver's side axle and bearing.
I don't have gages or measuring equipment calibrated to the level that I could detect a difference in case heads, but these sizing dies were all small base, I wanted the maximum case head reduction, for function and for safety in Garands and M1a's, so I tossed the dies. Once the metal is gone from the inside of the die, I cannot believe that the interior of the die is not larger, and thus, cases are not being sized as much as before. And there is nothing you can do to restore metal to the interior of a rusted die, at least for less than a new die costs.
I am being careful to take dies off the press after a sizing session, clean out the interior, and then wipe down with a rust preventing oil. I have come to the conclusion that while RCBS water soluble is an outstanding lubricant, it is not a good rust preventative.