The cement you should use in planters should be finely ground powder that will produce a consistent, smooth finish. Even though it is suitable, the larger grain cement and other chunky materials will not be as aesthetically appealing.
It would help if you also used dry cement because it is less caustic than wet cement. Wet cement contains higher levels of calcium oxide, which produces calcium hydroxide when it comes in contact with water. Calcium hydroxide accelerates the disintegration of your plastic planter.