decorators caulk

Hi I need to run a thin line of decorators caulk down my stair stringers to fill a small gap to the wall. I want a neat job and heard about the Fugi kit. My worry is it will score / scratch the paint on the walls (not an issue with baths / tiles). Any feedback?

It's high summer in the Mediterranean with outside shade temperatures hovering around +35C, and the inside temperature around+30C with 40% humidity. I am trying to wallpaper and am running into problems. The rooms are an old stone farmhouse, solid 800mm thick walls plastered with local lime...