permanent, but like all repairs only in the right circumstances.
I have done both, middle of tread as a permanent fix and one in the sidewall that was a get me home job..
Highly recommend Bike-Seal (www.bikeseal.com) it doesn't dissolve the tyre, being water based, and has a NATO ref no., (seals bullet holes or something) and apparently is approved by many of the blue light users. The stinger tyre deflator had to be redesigned as this stuff would seal the holes. The demo at an IAMc was very impressive.
