Replated Cylinder - Residue/grit after replate

[exbike]

I just got my replated cylinder back from Millenium. Looks like they did a nice job. It appears as if the technique used strips the paint off the outside - no big deal. The entire surface (except for the cylinder bore), including the gasket surfaces, has a hazy & gritty feel to it. Some of it has cleaned off in the sink, but the rough, fine-grit sandpaper feel is still there on all the surfaces.

What is that gritty stuff? Do I need to prep the gasket surfaces in any particular way before I rebuild?

Thanks! -Matt