Lynx is kind of right with the car body filler tip. There's a version for decorators which has been very popular for years (Decor Fill). I've gone off it a bit as I tend to use repair care instead, only problem with repair care is the investment in the system to get started and it's quite expensive if you're not splicing repairs with it. All the same it's the best system for this kind of repair.
Toupret has an alternative that you mix with water or another one that's 2 part, I've not used either of them but their products usually work better than most.
You'll likely want to get rid of a bit more of the rotted ends and use a hardener before filling.
Lynx is kind of right with the car body filler tip. There's a version for decorators which has been very popular for years (Decor Fill). I've gone off it a bit as I tend to use repair care instead, only problem with repair care is the investment in the system to get started and it's quite expensive if you're not splicing repairs with it. All the same it's the best system for this kind of repair.
Toupret has an alternative that you mix with water or another one that's 2 part, I've not used either of them but their products usually work better than most.
You'll likely want to get rid of a bit more of the rotted ends and use a hardener before filling.