Prep is most of the job, and it is the part that decides whether the floor is still down in five years.
It starts before anyone touches the slab. Moisture testing first, because vapor coming up through concrete will push a coating off no matter how good the product is. Joints get marked so they can be honored later instead of coated over. Drains get protected.
Then the surface comes off. Shot blasting or diamond grinding strips the weak paste, the laitance, and whatever has soaked in, and leaves a concrete surface profile with enough tooth for the epoxy to key into. Cracks and divots get filled with repair mortar.
Oil spots are their own problem. You keep treating them until water poured on the area darkens evenly instead of beading. If it still beads, the oil is still there, and the coating will let go right at that spot.
Planning a floor for a working facility? Here is how we approach concrete prep and commercial epoxy installation in St. Louis.