Do I use cerium oxide with the smoothing pad?

No.

Our smoothing pads are actually a diamond disk that will grind your glass.

Cerium is used for polishing your glass and is generally used with a carrier that has no grinding or polishing ability unto itself like the Synthetic Felt LP66 material

All you need to use with our Smoothing pads is water. They will give you a "pre-polish" finish from which you can use cerium oxide and a polishing pad to obtain your polish.