Updated: Aug 21, 2021
I've been spending some time recently with my K40 laser and etching anodized aluminum panels. The goal has been to achieve more colors than typical for a laser etched panel. Laser etching an anodized aluminum panel removes the anodization from that panel exposing an aluminum color. Then if you spray it with a "laser marking" material, you can etch it again, and it will bond a black color to bare aluminum. See below where I laser etch a black panel (left), apply the laser marking spray to the raw anodized panel (center), and then etch a red panel without the spray (right).
The center doesn't mark very well with black because I'm applying the marking spray to a fully anodized panel. But if you etch it first like in the far right etch above, then spray it and etch again where you want black, you get the image below, which is far better in contrast.
What this panel shows is that the black bonding of the marking spray is quite good but only in areas where there's solid print. The Fishbone image above has lots of dithering. The dots that make up the dither don't exactly line up after the marking, so depending on the angle you can still see bare aluminum, which drops the contrast of the black marking material. But in the solid areas it's quite good. Looking under the scope at a dot, there's an offset of the black marking material and the base aluminum spot. I'm not sure if that's a repeatability error in the K40 reprint, or if the black marking material shrinks a bit when hit by the laser. I'll experiment more in the future.
I have some ideas for laser etching of future products to get a panel that has 3 colors: red, black, and aluminum. Should be fun.