Are you getting enough of an engrave? I usually use 600/Full (Pro)/225LPI. I find that speeding up or lower resolution results in shallower engraves with less of a “cliff” that can bleed. The deeper engraves hold the Sharpie better.
Vary power will be problematic because the low end of tonality will have reduced power delivered to the tile. Using a dot pattern will make all of the engraving at the max power and I don’t get a mix of shallow & deep engraves that leads to erratic inking.
I’ve been doing 1000/Full(Pro)/450LPI … I’ll try slowing it down and see if it makes a difference. I’m cutting some more now, and I’m going to try baking them at 350 for 15 minutes. That’s been recommended on several blogs I’ve seen with just Sharpie and tiles. Sharpie recommends a fixative before the gloss spray too, so I’m putting that on my Home Depot list! 
I started with the 1000 speed and didn’t get as good results as 600 (even 700 is noticeably not as good - although okay). I hate slowing it down that slow because I hate 2 1/2 hour jobs 
But I like the results better so I suffer the slowness.
Art is pain, or something like that 
I have used two different methods, one of which is a little cheaper. I used the same 2x aerosol clear i use for auto touch-ups and just sprayed the tile without masking. No need to scrape later as it levels the engraved with the not and provides a smooth clear finish. The more expensive is you can use a airbrush and either cover the complete tile for the same results as above, or lightly blow a clear over your pattern. With airbrushes, you get what you pay for. A higher end like and Eclipse can paint as fine as eyelash results and in my opinion more user friendly.
I’m wondering now a year plus since this thread was most active how people’s tile sealants are holding up, and under what conditions. This fall I designed a set of engraved ceramic tiles that are in an outdoor application – at a pub as the table numbers glued to each table. Coloured them with sharpies, and sealed with a water-based sealant. Certain colours faded very quickly-- blues mostly, reds not at all-- of the ones in the sun in particular. The sealant was supposedly UV coated, and was suitable for outdoor use. I actually suspect the sealant itself is partly why the colours lifted, as I have one tile not sealed at all in my backyard that still looks great. I’m going to start a trial where I use different sealants on each tile and leave them all out in the elements. I’d love to know people’s experiences re: sealants (for regular sharpie colouring in particular, not the oil-based ones) at this point-- and I’ll do what I can to trial ones people have used. I relocated from US to UK earlier this year so couldn’t bring my spray lacquer type things with me, and can’t always get the exact same things here, but I’m working with what I can get.! First photos are how they looked before being installed… then you can see a couple where the blue in particular faded very badly, within 6 weeks.IMG_1468|500x500
