The top of this railing post is rotted and the post is not easy to replace. Luckily the rest of the post seems solid. What kind of wood filler product can I use to fill the rotted out holes before painting?
I understand that it won’t last forever. If it can last a couple of years, I’d be satisfied.
Use a high-performance 2-part wood filler. It's like automotive body filler but sets a little slower and is easy to plane/form/sand smooth after it sets. This is one brand that I like but there are many brands out there:
I use a "cheese grater" style plane (Surform) to get it flat, then sand smooth. Works great for repair work when replacement of damaged wood is impracticable.
Git-Rot is a product specifically made for this type of problem. While you should still clear out rotted wood as best you can, Git-Rot pours in as a thin epoxy and saturates the rotted wood you can't remove, soaking imto the good wood.
This gives you a solid foundation to apply an epoxy (2-part) wood filler of your choice.
