Have a body shop sand blast the rusty spots underneath and recoat with an epoxy primer (for corrosion protection) and then undercoating (for sound deadening). As for under hood. Either leave it alone or take the engine out, fenders off, hood off, and bumper off and remove rust. Spray rusty areas with an etching primer and recoat with basecoat/clearcoat if you have the resources. Or have someone else do it if you have money and are that concerned. Just a few ideas. Hope this was some help and feel free to ask any questions about the bodyman side of things. It's what I do.