Rotten wood: to fill or not to fill?

Question

I removed the paint from this wood, the top ring of a raised garden. While scraping I got to a soft part and easily removed this rotten wood, creating the hole in the pictures: it is 2x2x5"

Should I fill it completely with one of those things they sell to fill wood holes? is it too big for that? should I just fill it or first use the liquids to "harden" the wood? should I just spray it with something and keep it as is (it is kinda artistic, so I may go with that option, as long as it will not cause it to become worse faster).

if filling it, is it better to fill it with actual wood first and the rest with the filler (instead of wasting tons of filler?)

Lastly, will I be able to stain the wood, or since it will be a mix of wood and whatever filler I use, stain will not look good and I need to paint?

should I just remove the 4 woods (all 4 sides) and install new ones (all 4 so they look the same, not 3 old and one new).

Answer 1

Below quoted advice is pinched from here, How to paint a wood house?

If you encounter any damp, rotten, etc wood during the preparation stage, I highly recommend removing it. When doing so, cut 150mm (6inch) past the last sign of damage ie Get back to good, solid wood. Preferably re-patch with the same wood species.

In your particular situation (outdoors, horizontal surface) I would not recommend filling such a large hole with any kind of filler.

If you feel confident enough in your woodworking skills (or don't mind a challenge and want to learn something new), I say go with the above quoted method. If you were going to paint the wood, not stain it, I would be more likely to go with this method (paint can cover those little mistakes, where as stain is unforgiving).

That said it might just be easier (and cheaper) to remove the 4 pieces of wood completely and replace them with new timber, especially if you are going to stain the wood.

Answer 2

Looking at your pics, it looks like the other boards are not far behind. I wouldn't waste my time and money trying to fill, paint etc. Replace the boards with like material and start fresh.