I removed the excess wood with a Dremel-type tool and a router bit in a Stewart-MacDonald mini-router base. Serious routing with a Dremel is tricky, and you should work your way slowly to the required depth otherwise the router bit will be torn loose or start wandering around all over the place. If you haven't got a router, you could clean out the holes using a chisel. The results probably won't be as neat, but nobody's ever going to see it.
Now the body was ready for final filling, shaping and sanding. It's worth taking your time over this phase because the quality of the finish depends on the preparation. My timber had been fixed to the concrete fireplace with a number of large nails, so I had to fill all the holes using wood filler. I used the stuff that comes with a separate hardener, just like car filler. Once that was dry I sanded everything smooth, masked off the area where the fingerboard was to be glued on, and brushed on a coat of shellac-based sanding sealer.
Since I was aiming for a faded TV yellow finish I misted on a covering coat of the nearest colour I could find in Halfords - Rover Primula Yellow. Once that had dried I applied some dark grain-filler and followed this with some more misting colour coats. With just a hint of grain showing through the finish, I applied several coats of clear cellulose lacquer. The one I always use is Fiddes Bone Hard (www.fiddes.co.uk), which is available in aerosols. The finish had to sit for a week or so to harden before I could polish it up. I cut the surface back with 1500-grit wet and dry sandpaper (used wet) and then polished it, first with 3M Machine Polish, then Farecla G3. I used to use T-Cut but the ammonia in this product softens the finish, making the polishing stage harder than it needs to be.