How do I fill a groove or long ditch in wood to prepare to paint?

Wood putty and wood filler are effective for covering wide cracks on unstained wood in indoor projects.

You can also use epoxy to cover larger cracks, especially in outdoor projects and if you need a quick fix, use a mixture of wood glue and sawdust to fill in tiny crevices. Once you blend in and sand your filler, no one will notice the patched area.

I wouldn't use wood filler for such a wide gap. It would just crack and pop out with movement. Bondo (auto body filler) would work. Another option is to see if you can get some thin flat wood molding same height and width to fill the gap. Glue and nail. Then fill any small gaps with wood filler.

I would use the powder to mix with water filler. You can mix to the consistency you need. If you are going to stain, add stain in the mix (for future reference). For paint, just wait until fully dry. I've used Durham 's with great success.

If it were me I would not mess with any fillers. I would get a quarter round or a nice molding to fill the area.

Wood putty will crack. Use a piece of thin wood molding.

Cut a piece of molding to fit and fill it that way. You can glue it into place.

Bondo (a cement like substance) should work well to fill in the gap.

This may be helpful: https://www.youtube.com/watch?v=TwmV7absKns