Don't we hate those zippers that seize up and appeared corroded with coloured spots on them.
I considered WD40, but didn't wan't to stain the coat.
My wife's solution way to dab eucalyptus oil on it and leave it for a while.
Yes success.
My wife just loves to hear the words, 'you are right again dear'.

I will remember that I hate zippers

You will probably see white deposits between the slider and teeth. These are alkali salts from seawater. If you put a drop of acid on them you will see them fizz up and dissolve, and your zipper will then slide freely. Cleaning vinegar works, but slowly as it is quite weak. Something stronger like phosphoric (rust dissolver) or oxalic (any number of cleaning agents) will work faster and better. I have never seen these cause any damage to the zipper itself. I have started experimenting with a tiny spray of Lanox to see if this will prevent these deposits. Time will tell..

Cheers, Graeme