For this finish, we use a chemical smoothing process. Your model is put into a chamber with a solvent that seals the surface by removing the porosity and change the grey color to black. Due to the process, it smoothens the outside of your part, it’s possible that the inside is smoothed less. Check all the requirements for this finish in the design guidelines for an optimal outcome.
As with most post-production processes, extra labor is required and this will add 4 working days to the lead time.