Why does a tire patch leak near the lip, and how to properly patch the flat? I had a flat in my back wheel because the tire was worn out. I removed it and patched it as in these directions, except for the abrasion with sandpaper, which I never do. I put the tire back in and mounted the wheel, but it deflated again overnight. The leak is under the lip of the patch, which didn't completely seal, as in these pictures:




The hole was around 0.5mm so it is well covered by the rubber cement and the black area of the patch. I have patched many dozens of flats without this problem.

Why does the flat still leak? Is the orange area different from the black area, and does it have to be perfectly glued to the tire?
The edges of patches tend to come unstuck because you are attaching a stiff flat patch to a curved surface. To avoid this, make sure that the vulcanizing solution covers the entire area of the patch, apply pressure to the entire patch while it is curing, and wait a sufficient amount of time before reinflating the tube.
And as others have commented, abrading the tube before patching does also help get a good seal. Sometimes you’ll get a secure patch without it, but your chances are much better if you do it.