Frank,
Hit up Home Depot for some Duct sealer. Been using it and works well and is fairly cheap. Might even be pretty close to the GMK stuff. I think a 1 pound bar was two bucks. Look for it on the elec. fittings isle.
+1 on both Frank and Integra's posts. Balancing tires are one of the most important things a racer can do. Though I do get the mud on or in the wheels. Even still I think the benifits out weigh that.
I currently use the DuBro setup. Not the easiest to setup/use as some of the new style balancers out on the market. That is one of the reasons I have been looking to get one of the newer setups. But I am too cheap to get one and the DuBro I have works for me. I used to have one of those Top Flights and loved them. They could just barely do a 1/10th truck tire, but was probably one of the most accurate balancers I have ever used. I think back then it was about the only one around or at least the only one I saw being used. I would suspect the DuBro one has been around a while too.