When I started out attempting to engrave tiles, I ran tests of the cold galvanizing compound (CGC) vs. Rustoleum 2X white paint. I found the CGC superior for a number reasons, the strongest being that it does not disturb the shiny surface of the tile. I find when I use white paint, the tile is dulled and must be coated to restore the shine.
When I tried to do grayscale engraving on CGC, I received zero positive response. The two “colors” I got were splotchy and black. Then I tried the white paint tiles left over from my original testing, and I received this beautiful grayscale response.
So, I did a couple of the tiles above and thought I found the holy grail. Then I ran out of the original tiles (painted weeks before), and I could not reproduce the grayscale. After much testing and frustration, I found that cure time is one critical component. Twenty-four hours is not enough. Five days (I took a trip), is enough and I’m currently working through that stack. I expect the cure time will vary by conditions and be somewhere in that range.
A couple of additions. Imo, there is a missing light gray tone between the white of the tile and first gray in the scale, and the white can look funny/unnatural. I try to minimize the impact through image selection and image manipulation. Also, there is a banding issue I’ve yet to figure out.
A longer answer then you were probably looking for. The short answer is that the grayscale tiles above use white paint. All others used CGC.