I am about ready to grind , hone and polish a travertine floor. I realize I will need to fill the travertine. At what grit should I fill and with what type of filler should I use?

If you intend to grind the travertine flat I would grind and hone to about a 120 grit. Apply the filler after the 120 grit. There are several fillers that will work. There are products that are called traverfills or you can use a non-sanded grout. Either one of these materials will work. Be sure to overfill the holes since the filling material may shrink. Allow the filler too dry at least twenty four hours and then hone with the next grit. You may find that some fillers will pop out at this stage . If there are few holes I would fill them later after polishing. If there are numerous holes that pop out you will need to refill again.