Epoxy grout help

I have a fairly large shower and I cannot reach the entire pan without being on top of some of the pan. I am going to use epoxy grout but since I can’t reach the entire pan without pan, should I do it in stages, let it cure between stages so I don’t have to work over the top of grout that is not fully cured. Obviously it is the shower floor, so I will have to be on my hands and knees to both install and clean up the grout. Can I just use a piece is cardboard or plywood to protect the grout that has not been cleaned?