Everyone has cleaned a window, stepped back, and found it looked worse than before they started. It is the most common frustration people mention when they call us for the first time. The good news is that streaking has a small number of causes, and once you know which one you are dealing with it stops being mysterious.
Cause one: the glass is too warm
This is the biggest one and it is the reason we plan our routes around the sun. Cleaning solution on hot glass evaporates before you can pull it off, and what is left behind is the dissolved dirt, drying in place as a film. A south-facing window at two in the afternoon in July will streak no matter how good your technique is.
Our crews work the sunny side of a house early or late, and the shaded side in the middle of the day. If you are cleaning your own glass, do the same, or wait for an overcast day.
Cause two: too much soap
Most people use far more detergent than a window needs. A professional mix is a few drops of soap in a bucket, not a capful. Excess soap does not rinse off, it stays as a residue that attracts dust and dries as a haze the following day. If your windows look fine on the evening you clean them and hazy the next morning, soap is the likely culprit.
Cause three: the tool is worn
A squeegee rubber is a consumable. A nicked or hardened blade leaves a fine line of water on every pull, and those lines dry into the streaks you see from inside. We replace rubbers roughly every two weeks on a working crew, sooner if someone catches a blade on a frame edge.
- Run a finger along the blade. If it is not perfectly smooth and slightly tacky, change it.
- Never store a squeegee resting on its blade.
- Paper towel and newspaper leave lint and ink film. Use a clean microfibre for detailing instead.
- A dirty wash sleeve puts the last window's dirt onto the next window. Rinse it out.
Cause four: something was sprayed on the glass before
Ammonia-based sprays and anything containing silicone or waxes leave a coating. It looks fine when it is fresh, but it builds up, and when you finally wash it with a soap solution it comes off unevenly and smears. Homes that have used a well-known blue spray for years usually need one full stripping clean before the glass behaves normally again.
The same thing applies to rain repellent products people sometimes try on house windows. They are made for automotive glass and they do not wear evenly on a static pane.
What we do differently
On the outside we mostly use purified water through a brush on a pole. Purified water has had its minerals stripped, so it dries without leaving deposits and needs no detergent at all. On the inside, where poles are impractical, it is a light soap solution, a clean squeegee pulled in overlapping passes, and a dry microfibre on the edges.
The detail people miss is the last step. Edges hold the water that migrates back onto the glass as it dries. Drying the perimeter and the sill by hand is what separates a clean window from a nearly clean one.
Ninety percent of the streaks we get called about are warm glass, worn rubber or too much soap. None of the three has anything to do with the window itself.
If you have tried all of this and a pane still will not come clear, it may not be residue at all. Mineral etching from sprinkler spray and construction overspray both look like permanent streaking, and they need restoration work rather than another wash.
