wood is decyaing from water and ice/snow

Some bannisters have been exposed to the weather for around 10 years without any maintenance. The wood was treated so it's on ok condition but the top of the wood, the part of the bannister that probably has rain, ice, and snow on it for long periods has started to crack/fall out. I would like to sand these down and re-varnish them to protect them but what should I do with the top of the bannister? Can I fill these with wood filler and varnish them or would a better waterproof product be more suitable?