Late last fall I gutted a tiled shower I installed 20+ years ago. For numerous reasons I am just now getting it finished.
I was at the stage where I wanted to paint the ceiling and a portion of the shower walls above the tile. Then I am going to finish shower hardware and door installation. Then paint the entire bathroom. Sometimes I am able to cut in edges reasonably well. This wasn't the time. I was at a loss. I went youtube and found a couple videos on how to paint a straight line on rough surfaces using painters tape and clear caulk. Sometimes the information I have gotten off of youtube is successful for me, sometimes not. This worked extremely well!
It's also a way people paint an absolutely straight line on an accent wall.
The gest of it? You use painters tape to mark a straight line. Then you shoot a bead of clear caulk on the side of the tape where you want to paint. You wipe the excess caulk off with a wet finger, then using a liberal amount of paint on a brush, paint on the wet caulk. You remove the tape as soon as you finish painting the line. The wet caulk fills in the gaps between the low spots and the tape. I was stunned how well this to provide a straight line between a white and a bluish green Behr's Tasmanian Sea which are two very contrasting colors.