I’ve seen a number of videos recently of people restoring very rusted objects, like toys and an anceint historical knife, where the rust seems to have eaten most of the metal. However, after restoration, the object seems good as new and don’t seem to have lost any significant amount of mass even though you’d expect there to be nothing left. How is this even possible?
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When metal rusts, the rust only forms on the surface. When you see metal so rusty that it appears to have bark like a tree, what you’re seeing are extremely thin layers of rust that have piled up on top of each other with lots of space between them. This type of rust is a bit like pastry. Despite its size, there’s very little actual material there.
When restoring a part like this, the biggest problem is surface pitting. Rust can only form where the surface is exposed to oxygen, so the rust actually forms its own protective barrier to some degree, but oxygen is able to find its way in through cracks. The irregularity of the cracks leads to pitting on the surface.
To repair this, you either grind the surface flat, or you apply filler material. Which one you choose largely depends on the surface finish you’ll use. For example, if you intend to paint the item, you can sand it somewhat smooth, then use filler to smooth over the pits, then paint. If you’re going to polish the surface, you have to grind away material until it is flat.
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