First, clean using a clean rag and little liquid dish soap in very warm water.
Second, protect the surrounding areas you don't want to paint. Masking tape and some paper should do the trick.
The easiest way to sand is with a palm sander but this is limited to flat areas, You will still need to do some hand sanding to get rid of the chipped areas.
I suggest priming the entire railing if you plan on using an acrylic water based paint. If you an oil based paint priming is only needed where there is raw wood.
I don't know what aqau-marine paint is but I would use a good acrylic trim paint. Easier to work with and dries fast.