The requirements for mold storage vary greatly with the local conditions. I started casting in Miami Florida, and the combination of high humidity and salt in the air was a terror on guns and bullet molds. Rust Inhibiting Grease “RIG” was the order of the day. I brushed on the RIG on the molds while they were still warm, so the melted grease would soak into every crevice. The penalty for this was the time spent cleaning the mold before use, the mold had to be disassembled and the parts washed in solvent. The benefit was my molds stayed pristine.
The idea you can protect a mold by leaving a bullet in the cavity has been passed around for years. It is supposed to keep air out of the cavity and prevent rust. Unfortunately this is bogus information; a bullet in the cavity offers no protection whatsoever. Molds stored like this in Miami developed rust inside the cavities as well as outside.
Vapor phase inhibitors (VPI) are an easy way to protect your molds. They may not be sufficient in very humid climates, but in average climates they offer good protection for very little effort. The vapor of the chemical offers the protection, so you need a sealed container, and a plastic bag will work well. Vapor phase inhibitors do not affect mold fill out, you can use a mold directly from storage with no cleaning. VPI chemicals are available from Brownells.
My high school friend went to the University of New Mexico in Albuquerque. When I visited him his molds were stacked on a shelf, no oil at all, and they were in good condition, ready to use. In New Mexico the naturally low humidity protected the molds. I was impressed; he could just pick up a mold and start casting. When I returned home, I experimented with other storage methods to eliminate the cleaning. There are several ways you can store your molds, prevent rust, and yet have them ready to use.
You can provide low humidity by keeping your mold storage box warm. A small light bulb, or low wattage heater will keep the temperature up and the humidity down. This method is very popular in South America. In Brazil, gun storage closets had a small lamp low to the floor which was kept burning. Low wattage rod heaters, made for keeping pianos dry and in tune, can be used for gun or mold storage. These are much safer and longer lasting than light bulbs. I installed a rod heater in my gun closet in Florida.
Humidity can be reduced by using a desiccant. For this, you need an airtight storage box so the desiccant is not quickly exhausted. Most Tupperware-type kitchen boxes do not seal well enough for this use. Good quality military ammo boxes are the cheapest and most durable storage containers I have found. They have the added advantage you can put the molds in while still hot. I use the tall ammo boxes because they fit four cavity molds with the handles on.
Calcium chloride is a desiccant commonly sold in grocery stores. It absorbs water until it is a liquid, it is not reusable. The liquid is very corrosive, even small traces will quickly rust steel; it will even damage some stainless steels. Do not use it!
I use silica gel in cloth bags. I have to regenerate my desiccant bags about once a year. Silica gel is available in granules or small spheres with or without indicator. The granules should be spread in a layer one granule deep and heated for 1 hour at 250° F. The regenerated material should be immediately placed in a glass or metal container and sealed while hot. When cool it can be quickly transferred to paper or cloth bags for use.
The ability to use a mold directly from storage without the need to clean it is a tremendous timesaver and fully repays the effort needed to set up storage so oiling the mold is not needed. I now have to degrease my molds only once, after initial purchase.