I was told to prevent rust from forming in the mold cavity that I should leave the last casting in the mold while in storage. This didn't work. When I opened the mould several months after it was stored with the cavities still full of the alloy I found some faint signs of rust. Would like to hear your suggestions on the proper way to store bullet molds of iron or steel so that they do not rust. I've been told to never store them with oil on them or they will not cast well again. "OK"  I'm some what new to the casting game so I need your suggestion and guidance. Help!
                                              Sendaro

I store them both ways and it's the humidity that makes the difference. Find a way to keep them dry, such as a low watt bulb.

Frank

I leave them without a bullet in them, in my basement. I spray the cavity with liquid alox that I use to lube some bullets. When I'm ready to cast bullets I simply leave the mold sitting on top of my lead furnace to preheat the mold as the furnace warms up. Never had any issues with bad bullets using alox to protect the molds, and the warm up removes the alox, and gets the mold warmed so the first bullet is as good as later ones.

I leave a bullet in the mould and if the mould is not going to be used long term I use a Q-Tip and coat both the interior and exterior with Rapine Mould Prep and I have never had any rust issues. With the mould prep you don't have to clean the mould prior to using it. 

J.Louis

Kroil is great stuff! I've been using t for the past 12 or 14 years since being introduced to it in the benchrest shooting sport. The shooters of centerfire benchrest rifle use a mix of 1 part Kroil to 2 parts of Shooters Choise to clean their rifle bores with. It works well for cleaning powder/primer fouling from the bore of a rifle burning smokeless powder. Have used it on black powder but find there are better cleaners for that. Another use I've found for kroil is in knife sharpening. I use it on the sharpening stones and it seems to work very well for that. Also Kroil has a pleasant smell.
Thanks for the suggestion and the help. Sendaro

Paul Jones ( (You need to Login or Register to view media files and links)) recommends that you spray the mould cavity and exterior with gun oil (I use Rem Oil in the spray can) and then them wrap in plastic and store in a dry place.  I keep my moulds in the gun safe since I have a Golden Rod dehumidifier in the safe.  When you get ready to cast bullets clean the mould thoroughly with Birchwood Casey Gun Scrubber or Remington Shot Cleaner and let dry before casting bullets.  I never had any rust issues.  BTW - Paul says absolutely NEVER leave the last cast bullet in the mould.  Lead can and will absorb moisture.

I keep the humidity about 35-40% in my basement and do not leave any bullets in the mould.  I had one new Lyman mould start to rust before I had a chance to use it, but the blued finish was not very well done.  Any moulds made by Brooks, Saeco, & Jones have not been a problem.  I used to spray them down with 
WD40 or some other light oil, but then the de-greasing
issue can be problematic.  When a mould is not casting properly, I want to have a very short list of possible issues to deal with, so no more lubricants.
Keep on hav'n fun!
MikeT

Leaving a bullet in the cavity does make since. The moisture is in the air and the less air space, the less moister. Lead it's self can not absorb moisture (it's a non porous solid) so, from that stand point it does make since but, I've had light rust form with a bullet in the mold. 

I'm like some others here, I don't want anything but bare metal to pour the alloy into so, I just try to keep the moisture out of the air.

Frank