I have been using bondo molds for a long time and have good luck with the quantities I want to make. It is cheap and I found that it is best to make a couple of molds of the same jig.
The thing that destroys bondo is having your lead too hot and pouring them too quickly. If you are heating the lead in a pouring pot, you can controll the heat and using more than one mold you give it a chance to cool some.
Also -- when you make the molds be sure to soot the insides with a candle before using Them. This is a very important step many do not pay any attention to. It will allow the moulten metal to flow and fill the mold.
I have found that using anything but lead is a waste of time and effort. If you are making jigs for the ditch, you need the weight. I buy pure lead from the plumbing supply house and suppliment it with scrounged tire weights. Works pissa.
Just remember -- do not work indoors, wear a mask that will filter out the deadly fumes
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