Hello,
Iron to rust needs three things: iron, oxygen and moisture. My experience has been that iron relics deteriorating by rusting is increased with:
- Temperature change
- Environment humidity
- Rust and dirt that can hold moisture against the surface
- Surface coating
Number 1 seems to cause the biggest impact to them. Being iron, they have lots of mass and thus can't change their temperature as fast as the surrounding air might. This can cause them to sweat like a glass of iced tea on a hot and humid day if the air around them warms at a good rate. The amount of sweating is greatly impacted by the environment's humidity (#2). This has a bigger effect on rusty dirty shells (#3) as it will hold the moisture to the metal longer allowing for more breakdown of the iron.
The reason I mention #4, the surface coating, is that I've found that not all coatings fully protect the iron from moisture. Much of the above was gained from an experience where I loaned a number my shells to a small museum for display. The building gas heat was turned off at night and back on in the morning. The rapid heating of the room caused the shells to sweat every day. Within a time I noticed the rust forming on the surface of some shells. The ones that developed the surface rust were the ones that had a coat of Bri-Wax on them. Those coated in polyurethane did not rust; they just got wet every day. So even if you use Bri-Wax or something similar I would recommend a light coating of polyurethane to be safe.
That said, if the iron is kept in a controlled environment such as one's house then you should see no rusting or deterioration whether coated or not. I have uncoated and uncleaned iron that has not rusted a bit more because it's kept indoors with heat and A/C to control the temperature and humidity.
You are correct about the interior issue as well. I've seen shells sweat enough that a small pool of water would collect in the powder chamber. Coating the inside with polyurethane would help protect for more rusting but again if kept in a controlled environment this should not be an issue. While I'm too lazy to coat my shell interiors I do know a couple people who do (at least one of which is a forum member) and I think it is generally a good idea.
My thought on cutting shells: For common stuff in bad shape I don't have an issue with it but I do hate to see nice or rare shells that have been cut. That said, there is a wealth of knowledge that the half shells have given us regarding construction.