When confronted with foundation that reads "sheer" or "dewy," those with acne-prone skin have the same kind of involuntary, leg-jerk reaction you get when the doctor taps your kneecap (only maybe a little more subtle). After backing away, the next course of action is to layer on a heavy duty formula to cover all the red bumps up (then layer some more).
But, as you probably know but don't want to think about, that only clogs pores, which makes matter worse. So what's one to do? If you want to kill two birds with one stone, go for a makeup base with salicylic acid or an exfoliating agent that will treat your blemishes as well as disguise them. Or, at the very least, choose a formula that isn't too matte, too drying, or too greasy.