If you suspect grease and oil are contaminating the surface of the garage floor, these surfactants must be removed before coatings are applied. Unfortunately, grease and oils are commonly found on existing concrete floors. Dark spots are one such indication that grease and oil are present.
A quick test is to sprinkle a little water on the floor to see if beads up. Grease and oil will bead water or it may indicate a sealant or concrete curing agent is present. Another quick test is to apply duct tape to the suspected area and if the tape pulls away very easily, grease or oil are present.
To remove grease and oil apply a granular oil dry compound (or kitty litter) to the dark spots and areas where it is obvious that oil and grease are present. Allow to stand overnight and then sweep up the compound. Then, apply a good commercial oil and grease remover, following the manufacturers recommendations. An electric floor buffer with a scrub pad or a stiff bristle brush helps to work in the degreasing agent. Another good degreaser is a strong solution of TSP (TriSodium Phosphate). Be sure to use protective gloves and goggles. After scrubbing, rinse the area well and allow to dry. Pressure washers work well for rinsing if available.
The floor color should now be relatively uniform. If there are remaining dark spots, test with a drop of muriatic acid on the spots. If the acid bubbles, the floor is probably ok to coat. If the acid doesn’t bubble, the contaminents still exist and the floor must be degreased again. Repeat these steps until the floor is completely free of grease and oil.