How can I fill a balcony handrail that has rotted?

Nothing you can do but replace. Can't repair rotted wood. Sorry.

It sounds like the saddest thing to do would be replace. Maybe see if you can get a free estimate from a contractor, and ask them if there is an easy way to repair without replacing. You want to stay up to code and you don't want anyone falling through if you try to fix it and it should have been replaced

Use Bondo, it is usually for dents and gouges for cars, but will work on wood. Apply, smooth, let dry, sand, seal with several coats of paint.

Frankly, I would not consider this a diy project except by a skilled and experienced craftsman. There are things you could do cosmetically, but this is a life - safety issue. Don't take chances with anyone's life if it fails. I'd recommend a professional rebuild or replacement, and not by a basic handyman either. Other thoughts?

I totally agree with Sheryl. There are some situations where it just makes more sense to call a professional. I hate to ask this, but how would you feel if someone you love fell through a repaired post and got hurt falling?

Bite the bullet and replace as you don`t know just how much the holes have weakened the whole lot . Even over water it can`t be difficult using metal brackets ,try to replace with a hardwood like oak and make sure you treat it with a good few coats of at least linseed oil . And stop using pressure washer unless it is steam to clean.