Hi - I have a problem.
I'm making small parts from polyurethane into which I've stirred a small quantity of black pigment. The finish is good, apart from where the two halves of the mould are joined. There, I have to remove some excess material and then sand it smooth. That's OK except that where I sand the part, it's no longer shiny black but dullish grey. This is true however fine a grit I use (I've used 5000 grit wet and dry), or even metal polishing paste. I'm having to spray the parts black with paint which is a time consuming nuisance. Is there a way round this difficulty?