The best bare metal coating for non-exterior finished areas is "POR-15". It is available in a number of basic primary colours including black and silver. It dries epoxy hard. Ask "Google" about it. This would be ideal for your floor pans, inner rockers etc.
Secondly for corrosion resistance, use an oil spray like "Rust-Check". You need to get corrosion protection to go into the metal lap-joints.
HTH
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