“See how nasty that window looks?” A detailer posting for @detailingessentials, the account of a detailing training business, leans into the rear door glass of a red pickup to show a window covered edge to edge in pale spots. The caption promises “How to remove contamination from glass!!!”

He frames it as a test. “Now everybody on the internet’s gonna be like, I’ll just use a clay bar,” he says. “So let me show you.” What follows is three tries on the same window, one after another, under the bright shop lights.

Clay bar, then steel wool

The first try is the clay bar. He rubs a section of the glass with it, wipes it with a blue microfiber towel and gives the verdict: “That’s not gonna work. Clay bar didn’t work.”

The second is steel wool. He holds a gray pad up to the camera, “there’s your steel wool,” works it over the spotted glass and wipes again: “Yep, that didn’t work either.” Then comes his line: “This is why you hire a professional.”

The machine comes out

For the third try he picks up a small blue cordless machine polisher with a gray pad and runs it across the glass, then wipes the area with the towel. He turns to the camera with a raised finger and puts the question to viewers: “Based off of the results that I’m about to show you, which one worked better? Was it the clay? Was it the steel wool? Or was it doing it the right way?” The clip ends there with “Video over.” He does not name the polish, compound or pad he used, or the steps.

What the spots are

One common source of spotting on glass is hard water. The U.S. Geological Survey explains in its guide to water hardness that hardness “is caused by compounds of calcium and magnesium, and by a variety of other metals,” and that hard water leaves spots and film on glass and dishes, which the agency calls “not dangerous, but unsightly.” USGS classes water above 180 milligrams per liter, measured as calcium carbonate, as very hard, and 121 to 180 as hard.

Before you try this on your own glass

Wash and dry the glass first so you are judging only the spots that are left. Since he never names his product, here is one maker’s method: Meguiar’s says its Water Spot Remover is made to “gently polish away water spots from paint, glass, chrome, hard plastic and metals,” and that it can be used by hand or with a dual action polisher. The company says it is not for flat, matte or satin finishes.

Check your water. If you wash at home, look for a hardness figure in your water utility’s water quality report. On the USGS scale anything over 121 milligrams per liter is hard, so towel the car dry rather than letting the water dry on the glass.

Know whether your windows have film, and which side it is on. 3M’s care guide for its automotive window film says to wait at least 48 hours after installation before cleaning, to use glass cleaners only “as long as they do not contain abrasives,” to use a soft, non abrasive cloth, and to use only clean water because dirty water can carry sand and dust that “may scratch the window tint.” Keep steel wool, polish and any machine off a filmed surface.

Work one small patch at a time, the way he does one section of the window in the clip, and wipe it clean to judge the result before moving on.

Ask a pro what they will use. If you take his advice and hire it out, ask which product and pad the shop uses on glass, whether it is safe for the coatings and sensors on your windshield, and whether it will touch the tint side at all.