Just to add:
I mean, if someone goes to such lengths to produce prototypes, that is, using proper soldermask, silk-screen, multilayer, etc... Why not dojng it right to begin with? If you think your self-made prototypes are that important, or if you have such a large volume of one-off's, why use crap like a PCB mill? Use proper litho/film, expose the PCB, develop and etch. That will be faster than this milling crap, and with less dirt. If you have problems producing suitable stuff to expose the PCB's (i.e. like simple lase printers), go and grab an old Linotronic + RIP, and a developing-machine, and be done with it (similar old units available from Agfa and many others, you can get them for cheap used). Much more resolution than what you could get with milling any day.
Once you have that photo stuff for the PCB itself, re-use it for the soldermask. The only extra machine you need is a lmainator. And while at it, to produce a silk-screen you can use the same photo stuff again. Just check up these t-shirt printing things. It's a screen mesh where you apply photosensitive paint, let it dry, expose & develop, and then print thhrough. All you need is a screen with a finer mesh if you go for PCB legend stuff.
But really, spending thousands on these PCB milling machines, and then wasting lots and lots of time, only to have a 50/50 chance to get something useful ... really? All the work, dirt, consumables, etc., just for that?
And that, mind you, is only if you really need/want to do it on your own anyways.
Greetings,
Chris