Another vote for Evapo-Rust. When I got around to installing the new windshield frame for my '37 Chevy pickup, the frame had acquired surface rust, including inside the glass and wetherstrip channels. The Evap-Rust gave me clean metal again after a two-hour submersion.
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Lately, I've used it to derust a Buick nailhead water-jacket. They also offer a gel version of the product now to address the problem Kevin mentioned.
