It's called Trace Coating. A water based dye is applied and allowed to dry. Then sanding starts with, in the video, 120 grit. All the planer marks, drum sander scratches, glue spots, and other scratches and dings become readily apparent.
I just started using the process on the mesquite for the river table. Mesquite is a good bit harder than the maple used in the video. I started with 60, then 80 and 100. I'll do 120 and 180 and call it done. It's already dead flat and smooth as a baby's butt.
Here's Charles Neil's video about the process. He rambles a lot and, talks slow but, the information is really good.
Enjoy.
https://m.youtube.com/watch?t=4s&v=Ni2Y3vMtUSU
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