I have dropped using a sprayer (both with iso and water), but it makes a huge mess and I often get a drip or two (like the picture on the left). It is the only way I have been able to get anything slightly decent. I keep trying to paint on with straight water, pva glue, and most recently gun arabic). I can’t seem to get consistent results and always have banding from the brush. Basically it always looks like crap like the picture. I keep seeing people get beatiful results and I’m not sure what I could be doing wrong. I’m using a foam brush if that matters. It’s almost like the tile surface is so slick, it doesn’t grab the pigment, like trying to paint an unprimed surface. Any advise is appreciated! I’m finding this exponentially harder than etching slate. Cheers.
I had the most easy success using a sponge to ‘dab’ the gum arabic mixture on, rather than stroke.
The best quality I got was by ‘flooding’ the paint on, so it formed a big pillow of paint on the surface and then letting it dry on a perfectly flat surface (floating a flat piece of styrofoam in a dish of water is an easy way to get a flat surface).
The surface is very slick, you’re not wrong! If you still struggle after trying the above methods, it might be worth taking a piece of wet and dry sandpaper of a fine grit (say 800?) to the surface to rough it up a little before applying the paint. Do this underwater to catch the glass/glaze so you don’t end up with silicosis. 
Thank you. I will try a sponge and work on my technique. I had also considered using a very light abrasive, but i haven’t tried it yet.
For what it’s worth … I use mixture 1 tsp to 2 tsp methylated spirits (Denatured alcohol) to my American friends… stir well and lightly spread with a sponge brush… I put on about 3 coats waiting a couple minutes between coats… Have fun…
I tried the light abrasive, lost the shine of the glass on the tile where it was supposed to be white. Mine do best with a spray type. Anytime you’re using TiO2 powder, I think it’s a mess…
Just MHO…
Which paint do you like? I tried a couple but was unable to get a very black black, more like a gray