Fish eyes in the finished paint

I powder coat flat sheets of steel which are already painted but need to be painted a different colour,the problem is that on some sheets small circles appear where the powder did not adhere to I think there called fish eyes...what causes this problem and how can it be fixed.the sheets are rubbed down before hand using scotch cloth to roughin the surface and clean it.this only happens on pre painted steel sheets.

Ian, this sounds more like starbursting, your kv's are too high and need to be turned down. It will take some experimenting to get the kv's set correctly for a rework. If it is fisheyes, the parts aren't being cleaned well enough, you will need to wipe clean with a chemical. Alcohol works good.