They use a silicone wax type whatever goo to prepare the mould for casting, to avoid the resin sticking into the mould. In better places it is washed before the piece is sold, but sometimes not/not properly.
Wash the piece in warmish (not too warm as its gonna warp) water with dish soap, rub it a little bit with a sponge or an old toothbrush (depending on the size of the piece)), clear with clear water and its gonna work. I use GW promers without any problems.