Is there any benefit to steel plumb bobs over brass, or vice versa?

For those who don’t know, bobs are used to find the true vertical when building a structure. Generally, steel plumb bobs are less expensive than brass. Brass bobs, however, are better made – nicely machined in a full-body taper. Most even have a replaceable hardened-steel tip. With steel plumb bobs, you usually get a hexagon bar with a cone-shaped tip machined on the end. Except for the expensive and impeccable steel plumb bobs from storied toolmaker LS Starrett. Those tend to be lighter, smaller and narrower than brass, making them easier to carry in a toolbox. And their narrow profile makes them better suited for dropping close to a vertical surface.

Since the average steel plumb bob is lighter than brass, steel is less suited for outdoor use because a slight breeze can cause it to sway. And that brings us to our final issue: which plumb bob settles faster? It’s not a matter of material, actually. A masterful study by tinuum (a newsletter for plumb-bob collectors, who actually exist) found that a heavier plumb bob on a longer line takes longer to settle than a lighter one on a shorter line, due to the bob’s tendency to act like a pendulum. But here it gets complicated. The brass bob’s classic taper moves its centre of gravity higher above the floor, effectively reducing line length and offsetting the pendulum action created by its higher weight. So which should you use? If I have a choice, I go with brass for outdoors and steel when I’m inside. You probably painted on a cold surface. Moisture in the air condenses on the colder part of the surface, dust clings to it, and mildew forms, causing the lines that you see. I’m guessing that your lines are along drywall studs or ceiling joists. This is because of a phenomenon known as thermal bridging, in which the framing lumber conducts cold from the outside wall to the interior drywall.

Two things cause this: you have inadequate or poorly installed insulation, or you have an air leak on the outside wall surface that’s letting cold air into the wall cavity. Most likely those shadow lines were there before you painted, and they became more apparent with the new paint. If you added brighter paint, the shadow lines will provide even more contrast to it.

Unfortunately, there isn’t an easy solution. First, you need to hire a specialist to inspect the house with an infrared camera, revealing exactly where the house is leaking energy. Based on that information, you know where to seal air leaks and install insulation. After that work is complete, apply a primer sealer to the wall to permanently hide the discolouration. Follow that with the same topcoat paint that you used in the rest of the room. Painting the wall from corner to corner will most likely make it indistinguishable from the rest of the room.