Tire Age or Wear - The saying is "You can't make a silk purse out of a sow's ear". This applies to tires as well. When a tire is over five years old and shows signs of cracking or dry rotting, it should not be repaired.
The tread may well be "like new" but the integrity of the rubber has been compromised. The same thing can be said of a tire which is at the tread wear indicators, in which case, it does not merit the expense of the repair either.
Un-repairable Puncture - There are a host of conditions that exclude a tire from repair. Some of them are: an injury over 1/4 inch; a puncture so close to a previously repaired area that the patches would overlap; an injury beyond the repairable area which excludes the shoulder and sidewall, and a tire with a damaged inner liner usually the result of running the tire flat or with too little air pressure.
Run Flat or Sealant Treated Tires - Some run flat technology tires cannot be repaired and at the very least the manufacturer should be consulted prior to attempting a repair. Some tire sealants, commonly called fix-a-flat, make it extremely difficult to repair a tire that has been so treated. At the very minimum extra expense will be encountered to clean out the inside of the tire. In many cases these tires have been run with low tire pressure and have damage in addition to the initial puncture and cannot be repaired.
While these are certainly not all the circumstances that may exclude a tire from being repaired, they can give you an insight into the hurdles encountered by tire repairmen.
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