Don't Use The Grout, Its Not The Same As Thinset Mortar, Mix Up Thinset Thick, Like Peanut Butter, Find The Dips And Skim Over Them, And Then Skim The Hole Floor, You don't Have To Go Very Thick, Maybe Not More Than 1/2 " May Take The Dips Out And Be Good Enough To Lay Tile On.