Can I use this product on an MC metal clad electrical line that is rusting underneath the house?

Yes, Loctite Extend Rust Neutralizer can be used to help preserve surfaces and will slow down the activation process on the cable. The MC metal clad electrical line has a flexible armor usually made from aluminum. Aluminum does not rust but it does corrode. On the aluminum surface, the Extend Rust Neutralizer will not turn into a black coating. Rather it will dry to an opaque protective coating. Free iron must be present for the product to convert to a black coating. Keep in mind that a solvent-based topcoat is required after the Extend Rust Neutralizer has dried.