I asked this on dirtoval.com and got this in an email hope it helps.
best silicone was from GE it was clear I used a drill press chucked up an old rc 10 rear axel and then put a wheel with foam on it run the drill press on a slow speed put a little on tire at a time smoothing out with a wet finger let it dry and repeat has to dry over night I made a thing to hold tires and wheels so that they didn't thouch anything while the silicone was wet. Put as many coats on as you want and when you start seeing holes in them do it all again. tires really stick on a hard packed clay track when its watered properly
best silicone was from GE it was clear I used a drill press chucked up an old rc 10 rear axel and then put a wheel with foam on it run the drill press on a slow speed put a little on tire at a time smoothing out with a wet finger let it dry and repeat has to dry over night I made a thing to hold tires and wheels so that they didn't thouch anything while the silicone was wet. Put as many coats on as you want and when you start seeing holes in them do it all again. tires really stick on a hard packed clay track when its watered properly