Following all of the good advice here, I had another shot. I followed @Julien’s advice and surfaced the wood to ensure flatness. I made one pass with a 24mm surfacing cutter on both sides of the wood. The pass was 0.01mm on either side and it was sufficient. When looking closely at the wood (poplar again) the centreline join is now quite good and one may miss it on first glance if you did not know where to find it.
I had also followed Julien’s advice to cut the pattern twice. I did this to a depth of 0.2mm (0.0078") and I was reasonably happy with the image. I suspect that I was too rough with the soft brass brush and took out more than just a few of the fuzzies.
The next image shows the pattern covered with paint. These are from a range of non-toxic paints and varnishes that have been tested and approved for children. The paint dries super quick as does the matt varnish which I had applied before the paint. The paint is a chalk based product and has no VOCs or any nasty chemicals. This paint covered pattern dried in less than 10 minutes.
For people who have an interest in non toxic paints for kids toys, furniture and rooms; below is a link to the Danish company and the relevant data sheets for all of their products. The colours are available in very bright primary colours and pastel shades. It covers wood in a single coat without the need for primer and I used their matt varnish as a protective overcoat. Highly recommended product but quite expensive.
Finally, the finished job which is a fair proof of concept but not quite as clean an image as I had wanted. I was aiming for a technical drawing like feel but did not quite achieve that look. My ‘very light’ sanding was uneven and I removed a little of illustration by being too heavy handed. On the other hand… this is the start of another skill set development pathway. A big “Thank you!” to all of the people in this thread, who had offered me some sound advice.
I have some pre-stained wood to try the method outlined by @fenrus so another carving and a different technique should be ready for display by tomorrow.
A technical aside is that I checked the belt tension on each rail and they are still not reading differently to the original 135Hz. I had tuned them following the installation of @neilferreri’s belt tension mechanism, ably printed for me by @LiamN. (Thanks guys. They are working really nicely)