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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:

Picture of top of patch Picture of lip of patch

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?

miguelmorin
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    If you didn't sand the tube there's probably a slight air gap along the seam... – DavidW Nov 11 '19 at 21:04
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    Sanding the patch area really shouldn't be considered optional. Innertubes are coated with mold-release compound that gets in the way of the vulcanizing agent reacting with the rubber. You can abrade it off or wash it off with soap and water. The sandpaper probably also creates a little more surface area for the vulcanizing agent to react with. – Adam Rice Nov 11 '19 at 21:12
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    Another important step is to thoroughly press down the patch. I place the patched tire on a flat surface and use something like a smooth edge of a tire iron to rub across the surface, going back and forth to cover the entire patch. – Daniel R Hicks Nov 11 '19 at 21:38
  • (Er, I mean "patched tube".) – Daniel R Hicks Nov 11 '19 at 23:08
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    It should be mentioned that another cause for such a leak is that there is a hole in the tube near the edge of the patch. This can very easily happen when the puncturing object "dances around" a bit and pokes the tube in several different spots. – Daniel R Hicks Nov 12 '19 at 03:16
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    Rip the current patch off, bin it, and try again with a fresh patch. This time follow the instructions to abrade the tube, which you have to do even more given the old glue. Also give the wet glue 5-10 minutes to "set up" on the tube, before you slap on the patch. – Criggie Nov 12 '19 at 08:37
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    Also, given your tyre is "worn out" expect more of these until you replace the tyre. – Criggie Nov 12 '19 at 08:38
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    Also apply glue to an area larger than the patch so the whole of the patch is glued down, including the orange part. Trying to save glue is a false economy, as you just found out. – Argenti Apparatus Nov 12 '19 at 12:05
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    You answered your own question by admitting that you don't bother with the abrasion part of the instructions :) – Henry S Nov 12 '19 at 14:26
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    At this point, I would just get a new tube. Trying to re-do the patch will probably just make the leak worse. – Mike Baranczak Nov 13 '19 at 00:25

3 Answers3

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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.

Andrew
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Sanding the patch area really shouldn't be considered optional. Innertubes are coated with mold-release compound that prevents the vulcanizing agent from reacting with the rubber. You can abrade it off or wash it off with soap and water. The sandpaper probably also creates a little more surface area for the vulcanizing agent to react with.

In the case shown here, the vulcanizing agent doesn't cover enough area under the patch, as Argenti commented, so the edges don't have a chance to adhere. In my experience, the tube of vulcanizing agent usually dries out before I've used all the patches that come with a kit, so there's not much point being stingy with it.

Adam Rice
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I took all of the above suggestions: I removed the previous patch, applied glue generously, let it sit for 10 minutes, and applied the patch. I took the "thorough pressure on the patch" too literally and used clamps; the tube adhered to the clamps and had another puncture, which I fixed in the same way without the clamps. It all works now.

I had used too little glue because the patch had the width of the tube and the glue on one side didn't entirely contain it. I started using smaller patches instead.

All my punctures have worked without sanding, except maybe three occasions similar to this one where I ended up putting a patch on top of the previous patch. @Criggie suggested that one could rip the previous patch and that works.

miguelmorin
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