For balancing 1/8th scale tires I use either self-adhesive lead tape (like at the golf store but I get a larger roll from McMaster-Carr) or 1/8" solid wire solder. I squash the solder in a vise to make it into a flat strip and attach it to the wheel with shoe goo. Once dry, it does not pick up any dirt like clay does.

I use the flattened solder when the wheel is more out of balance than can be corrected with a reasonable amount of the lead tape. Also, I cut the wire solder into pieces no longer than 3/8" because longer pieces tend to come unglued because of wheel flex.