Tire should be replaced. It is worn thin, and you will get more and more flats. Unless your time is worth nothing, or you are enjoying this, or you literally don't have $25, it is time for a new tire.
Sometimes a wire embedded in a tire casing will cause a slow leak when it is pressing into the tube, but the tube won't leak when it is out of the tube. So if you insist on using this tire, you need to take the time to search for the wire or other sharp thing. Also check that the rim strip is covering all the spoke ends.
Shortcuts don't work well when fixing flats.
Last edited by jyl; 12-22-15 at 02:18 PM.