We washed them first, based on advice I had seen in other forums. Simply had a bowl with warm water and a little dish detergent. Dropped them all in, gave them a quick scrub with a toothbrush, then dropped them in a 2nd bowl of clean water to rinse. Dried them with paper towels. What I had read, is that the injection molds sometimes have a coating to make it easier to eject the minis. This residue can interfere with painting. We were going to paint some reaper Bones as well, so we washed a big batch.
However, my 11 yo son started painting his Samurai's without washing them and he didn't run into any issues.
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This was our first time ever painting miniatures so what we did may not be correct.