Of the ones I've tried, I've found "Magica" ( www.magicarustremover.com
) to be the strongest and easier. I'm pretty sure it's oxalic acid. Get
the little tube which is a gel. This is particularly easy to dab onto
stainless fittings and screw heads without getting the stuff
everywhere. Wear rubber gloves anyway. (I have no relation to the
company).

Practical Sailor also just ran a test and like Star Brites rust remover
a lot which is also oxalic acid.

Carl