Two tools will make our work a lot easier. First, a small curved finger plane is very useful for carving the top block; for most of this work, the plane cuts best working uphill, that is, from the tip of the block back toward the mold.
The second tool is a Veritas low-angle spokeshave, available in Canada from Lee Valley Tools. This tool can be set closely to take very fine cuts, and it works well across the grain as well as on end grain--which we have to deal with down at the bottom end of the mold.
These next two photos show the kind of progress that you can make with these tools. The first shows an overall smoothing both on the mold and on the top block; the second photo shows how I've used the spokeshave directly over the cross section to carve things down in a controlled way.
Once we start doing that, however, we will have to say goodbye to our cross section lines, because they are going to be (mostly) carved away. We'll need to mark our rib location points so we can find them again as we work. Here's how.
Take a sharp pin, and prick each rib point deeply.
To lay out the rib lines on your mold, you need to connect the dots--all of the rib points on each of the cross sections in your mold--from the very tip of your top block all the way back to the bottom end of the mold.
The ribs are laid out on the face of the top block using a template derived from your cross section drawing. Notice how I've used a chisel to chamfer down to the line here, just as I did on the cross section blocks of the mold.
Now, to connect the dots and make an accurate rib line, you'll want to make yourself this very handy tool. Take a thin, flexible strip of plexiglass, between about 15-20 mm wide, and about 70 or 80mm long. Scribe a straight line down the center of it, and every 10mm or so, drill a small hole for your marking pin to fit through. Take this marking strap to your mold, and set the scribed line over the dots of one of the rib lines from the top to the bottom (you can fix the strap in place with pins).
When everything's aligned correctly, use your marking pin to prick through the holes into the mold. Then remove the marking strap, and tattoo the pin holes with a felt-tip pen.
All that's left to do now is to connect these tattooed dots, and I do this with a sharp, hard pencil and a flexible curve laid on its side. (You'll need to cut away the inking 'lip' on the side of the flexible curve for it to lie close to the mold so you can mark an accurate line.)
Here are the rib lines, neatly laid out.
I like using the marking strap and the flexible curve, rather than just the flexible curve, because the marking strap ensures that I'm creating a single, continuous line. Because it's too short to reach over the entire length of the mold, the flexible curve by itself can only give segments of a line, with no real guarantee that the segments will align well.
Creating the Rib FacetsNow that the lines are laid out, the mold's facets can be carved. This operation is pretty straightforward: all we're doing is removing material between the rib lines. As long as you can plane, and scrape, between the lines, you'll be fine.
For removing the bulk of the material, you can use your small finger plane.
The part of the process that we've just begun, of carving and refining, takes a long time, I've found, and a lot of concentration and stamina. Have a good sleep and eat a good breakfast, and I'll meet you back in the shop in the morning.