loosening fittings (hard water)

Have a customer that has two showers both affected by hard water deposits; I told him to remove the heads and soak them in vinegar for a few days, do them one at a time but.....he has tried to remove the head from the shower arm and is afraid of damaging the finish or maybe splitting the head. Is there something we could spray or brush on the hard water area of the fitting (like you would on a badly rusted fitting) that might soak in and allow it to break loose? Then we can clean up the shower arm, use Teflon tape to help retard further formation and hopefully get the head and fittings cleaned up good.
I am sure one of you great plumbers have bumped into this issue over the years!

loosening fittings (hard water)