I used Dunhams wood putty on my strip canoe, which had gaps much bigger than what you're talking about. As suggested, it sands a LOT more easily than epoxy/woodflour mixtures. Sanding it is similar to sanding an expoxy/microballoon fairing compound. Though it's been a while since I bought any of the water-mix wood putty, it must cost about 1/78th of what any expoxy/combo mix you could come up with. If I'd have bothered to tint it, it might have matched the cedar in my canoe better, but after a dozen or so years, the putty is still holding up fine.