Every so often someone asks what to use to stop solder from flowing when making a new join. We seem to, as a group, resist using yellow ochre, and there is good reason for that. I actually like it, as long as I don’t immerse my ochred piece in my pickle after soldering. Ochre is an iron-based ore, and will contaminate pickle.
Then peeps suggest Wite-Out, which is fine to use as long as you use the water-soluble product and not the one that’s made with solvents. The solvents are the problem, as they create truly nasty fumes that can damage the only pair of lungs you’ll ever have.
Ronda Coryell has made a product, Masking Mud, which I love. It’s much easier to clean up after use than White-Out or Ochre, produces no nasty fumes, and, yes, it does what it’s supposed to do, stops solder flow when you don’t want solder to flow. I am not affiliated with Ronda and/or any supplier of the product (just sayin’ in the interest of full disclosure).