I cannot stress this enough - polish your preparations - especially your margins. If you do not, you risk having a less than ideal fit of your restoration. You see, the margins of the preparation are milled in a completely different manner than how the rest of the restoration is milled.

On the margins, the machine does everything possible to try to minimize overmilling, which is simply milling out the porcelain to get the restoration to seat. If we allowed this to occur on the margins, guess, what - you get an open margin.

Now the other important point to remember is that bur that mills the margin is 1.4mm in diameter. Relatively speaking, its a fairly large bur. Now if you have a spot on your margin that is smaller than 1.4mm, then obviously your bur will have a difficult time milling.

I recommend polishing all preparation margins. My favorite tool is a slow speed round bur but you could really use anything - hand instrument, white stone, finishing diamond - anything that will give your margins a glass smooth finish.

If you take this one step of polishing your margins, the fit of your restorations will be unlike anything you have every seen.