Actually, now playing a bit more, I can relate to that. as long as you do “brute force”, the slight uneven coverage of the sharpie does not matter, but if you try to fine tune laser power, an uneven coverage suddenly shows in the results. So I tried a bit with paint (tried tempera and acrylic, just because I had them available), which makes it easier to get a fully opaque cover. My conclusion is, it is not really important which cover you use, as long as it absorbs all laser power, and it does not need to be super evenly distributed, as long as it is fully opaque. Still, avoid thick globs of paint, these absorb laser too well and nothing reaches the glass-paint-transition-zone.
Using Lightburn’s offset fill method (which IMHO is already reason enough to purchase Lightburn), 10W at 400 mm/s and 40% I get nice results (perhaps 30% might have been enough). I do not succeed to achieve the silky effect I was aiming for, and I guess that’s because the glass shatters in too large flakes for that, but still, the result is pleasing and I guess I’m ready for the final project, i.e. engraving the spice jars.
You may ask: tempera or acrylic? I’ll go for acrylic, it smells better and after the process it kindof peels off easily, while tempera leaves marks sometimes. Acrylic also left a bit of sooty residue in places, but I found that a wire brush is good to remove these, and my fear that this would leave scratches on the glass did not come true.
Here’s how it turned out (text is 5cm wide and nearly 1cm high):
