I've been using toilet paper cardboard tubes that distort nicely to the rim radius. (Or a little more radius so the glue squeezes toward the edges where I really want it. Clean, no cleanup and reusable. Now, that acid brush approach looks attractive. Better control of amounts and thicknesses.
DiabloScott, can that syringe be used later for additional tires? And if so, how do you store it and keep it working? Any neat tricks?