+1 for boiling water, I`ve been using it on old carburetors for years. My favorite for rust removal is molasses, sold in agricultural stores as a horse feed supplement and costs very little. It`s not fast but it does the job.
50/50 oil and kero is as good as any of the products sold as penetrating oil and costs a lot less, it`s also perfect on oilstones when sharpening tools.
Bob