I'm finishing flat hardwood boards and I'd like the finished surface to perfectly smooth. I'm starting the project with walnut and trying to find the best way to fill those tiny, hairline grain cracks. Should I just use colored wood filling crayons? Is there a filler that's best? I've tried flooding them with CA glue. That seems to work, but I'm worried about whether the CA will react well with the final finishing substance (e.g., block oil, Rubio Monocoat, Osmo Polyex, etc.)
Is there a best practice for this sort of filling?