To answer your first question, it is advisable to allow the acne cream approximately 15 minutes to be absorbed into the skin before applying your moisturizer. This allows it time to penetrate the skin and start working.
Moisturizers are designed to work by preventing water loss from the surface of the skin and trapping it in the outer layer of cells. The greasier or oilier a product is, the longer this effect will last. Because you need to use a light, oil-free moisturizer, this effect will last for only a short time, allowing the skin to dry out again.
You can apply the moisturizer as often as you feel that your skin needs it – which can be as often as every hour.
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