How to clean / fix rusting heatilators? esp. in bathroom, foyers?

I am not sure what is rusting in your heatilators, but if you can remove them, Rustoleum is great paint to apply and repaint to prevent rust! If there is more than just cosmetic rust, you may need to sand it first or replace it if it is too deep of rust.

light sand,clean ,prime with a rust proof barrier,allow to dry and paint with a metal paint

Rub down and re-paint

I do my heater covers during spring or summer, when they aren't being used. Clean them really well (you can do that now). If the rust is superficial, not damaging the metal to the point it isn't strong enough for what it is used for, once you are able, take the affected parts off and take them outside. A wire brush will help you get to bare metal pretty quickly, and/or sand it with a block of sand paper or an electric sander. Be sure to wear safety glasses. Once you get to bare metal, use a primer and then the final color you want, following the directions. It is better to spray lightly several times, then to have it go on too thick. If your covers get really hot, check the label of the paint you want to use, to make sure it is the right spray paint for this project. I like Rustoleum paint, but other paint can be used. The important thing is to get to the bare metal, prime and paint, outdoors with good ventilation.