A repair plug insertable by a pneumatic gun or other tool into a puncture formed in the casing of a tubeless tire, thereby to seal the puncture. The plug is constituted by a core of cured rubber coated with a tacky layer of vulcanizable rubber which, when subjected to the heat produced by a running tire, vulcanizes in place and bonds to the tire casing. Before being inserted in the puncture, the plug is coated with a temporary detackifying compound which lubricates the passage of the plug through the inserting tool. The compound is dissolvable in rubber when subjected to the heat of the running tire so that it disappears in the plug and the bonding action of the tacky coating thereon is permitted to proceed.