Powder Coat no longer sticks to Tungsten Carbide

Have been successfully powder coating tungsten carbide materials from several suppliers for years. Our newest supplier has sent us materials that we cannot get the powder coat to stick to - it looks polished enough. I am wondering if this is an issue of maybe being TOO polished - not giving the coat anyhting to key to.

I understand that the "polishing" of WC is a field of study in itself, so am looking for any hints or clues here.

Could be the new guy coats his material with something that isn't being removed by your pretreatment system. This new supplier obviously does something with his process that the other suppliers don't, seems like all you'd have to do is ask him what it is.