You know that most racing tires leak air right through the rubber right? If they hold air for an hour that’s my minimum standard for sprint racing. Have you tried wire brushing or using 150 grit sandpaper. If you have some way to spin the wheel it makes it easy. You could also try a very thin layer of epoxy or JB weld.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience