Yes Pete, I hate silicone. The No More Gaps stuff is acrylic, stays flexible if there is joint movement, and paintable. It doesn’t sand well though.
I used that Plastibond bog before @JeffH and it is indeed very good. Very expensive and maybe overkill for this particular application?
Yesterday I painted the boxes with undercoat so I can see properly where needs to be filled - and pretty much 90% of all finger joints need filling on this box. It’s quite a workout, and with the screw head protruding it is a pain to sand. I might have to take the screws out, fill and sand, put the screws back in, then paint. Phew…
If I don’t fill these gaps properly, water will penetrate and the box will rot. (I’m not a copper naphthenate fan)
I don’t know why it is that bad… maybe the timber wasn’t dry enough and shrunk by the time I put it together?