Rust is the open air oxidation -- reaction to oxygen -- of irons and its alloys, like steel. The rust, which is an iron oxide, puffs up and sometimes flakes off the metal surface, sometimes separating two joined or sealed parts. However, rust can also stop objects from being removed from each other -- such as a nut and bolt -- and it must be detached from the metal surface before the parts can be loosened.
Brush the two rusted parts with a steel brush, to remove as much rust as possible. Brush around all surfaces of the parts.
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Tap both parts lightly with a hammer from all sides, to help dislodge any rust that cannot be reached with the steel brush. Attempt to remove the parts from each other by forcing a chisel between the two parts and knocking the chisel with the hammer, or in the case of a nut and bolt, place a wrench around the nut and attempt to turn the wrench counter-clockwise.
Apply rust-dissolving spray liberally around the join between the two rusted parts, and wait 15 minutes for the spray to take effect. Then attempt to separate the parts from each other, as described in Step 2.
Apply heat to the rusted join between the material surfaces using a propane torch, or if possible, place the rusted material in a freezer to cool the material. Heat will cause metal to expand and cold will cause metal to contract, each helping to dislodge the rust. Tap the material again with the hammer, and attempt to separate the two parts as described in Step 2.