I read on the forum of some folks using a product called Evapo-Rust to get rust out of their moulds. I have a problem with one mould in particular, so I decided to give it a try.
Since North Dakota isn't exactly known as a humid environment, I typically cast in my shed and let the moulds cool down on their own overnight, recovering them the next day. This has worked for me with Saeco, Modern Bond, Ideal, Lyman, RCBS, and Cramer moulds. I have never had an issue. In October 2008, I purchased a new Lyman 4 cavity mould (Cut Aug 08). From the first time I used it, the mould flash rusted but the cavities remained in good shape.
When I checked the mould a few days ago, I noticed some rust in the cavities. I decided to give Evapo-Rust a try. I am almost ashamed to post these before pictures.
Both Blocks
I decided to do the worst looking block of the two. This would give me (hopefully) the most drastic transformation.
The cavities of said mould.
This is the product package and the mould in question.
Using a plastic storage container from Dollar Tree, I placed the mould on the bottom and poured enough Evapo-Rust in to cover the mould by about 1/4".
The results: After 1/2 hour, this is how the mould looked. I would say that 90% of the block was amazing. However, I still had a slight rusting on the exterior of the mould and the cavitites still had a little rust. I rolled the it over and placed it back in the solution.
So I left the mould in the solution overnight. This is how I just took it out.
But the real success was in the cavities. The block looks amazing. I can't see any pitting at all.
This is how the worst looking block turned out. It looks fantastic.
After taking the pic, I put the other block, sprue plate, and hardware in the solution. I coated the first block with Kroil.
From my experience, I would absolutely recommend this product. I believe it cost around $7.00 at O'Reiley's Auto Parts. Use was absolutely simple: Put the product in the solution overnight, rinse the next day, and coat in oil. Doesn't get much simpler than that.