Chris, I've used "Rust Eater" from Autozone with good results, others have recommended PB Blaster for similar use. One thing I would heartily recommend is getting some anti-sieze compound before you re-assemble the housing. If you can't get the bolts out with an easy out of vise-grips, try heating the housing around the bolt holes with a propane torch, the aluminum will expand more rapidly than the bolts and may loosen up in the holes. HTH.
Ron
Ron