Have you tried to remove the ash yet and if so, what are you using? If not, or if you have and believe you jacked it up, look for a brass wire brush with soft bristles.
Used a big wire brush and went (mostly) with the grain. Brush measures roughly 3x7 inch, so it spread out the down force fairly nicely. Accidentally went cross grain a few times, but if I wanted perfect I wouldn't be burning it.
Vacuumed it off with a soft brush attachment
Then started wiping with paper towels.
Took a while to get the oily char off the surface, then cleaned with mineral spirits.
Used half a roll of paper towels wiping with and without spirits.
Coated the underside with Minwax clear semi gloss floor polyurethane this afternoon.
You can see some imperfections in the wood on the underside.
I hit the top before bed.
Pictures really don't do it justice, I'll take another with natural light in the morning.
A few notes.
The fire will burn the glue out near the surface. You're not going to get a writing surface unless you use an epoxy or fill the gaps somehow.
Beware of warping, the heat will cause the center to rise. I flipped it and applied heat to the other side so it would cool evenly. It was also 40 degrees F outside, so the temperature differential was greater.
These butcher blocks will have knots and imperfections on one side. Deal with it.
I haven't touched this with sandpaper. AT ALL. Right out of the plastic, fire, wire brush, cleaning, poly. I would like some shelves above the desk, so this piece may become shelves at some point, but I'll see how it works.