To prevent acne, first of all cut back on any high GI carbs, thus dextrose/glucose/fructose and maltodextrin after a workout shoud be avoided, and switch to low GI carbs such as Scottish Oats.
In regards to whey protein, unlike MP some manufactures use ion exchange whey protein isolate, which has been proven to contain denatured fractions of whey, which will cause acne and furthermore will be a waste of protein.
I personally use scottish oats and a combo of egg white, whey protein isolates and concentrates and casesin. this sustaine release formula will not flactuate yor insulin levels and cause acne. Furthermore try addind fish oils and flaxseed oil to your diet which besides numerous other reported benefits will reduce acne and improve overall skin condtion, such as scars and stretch marks as well. As tribes which consume loads of fish never get acne.